Preliminary Once-Off Steps

NB! Before using the action it is imperative that you set-up your black-point, white-point and gray-point values correctly. To do this perform the following steps (note that this is a once-off step and does not need to be repeated when you run the action in future)

Open up your image to be edited in Photoshop and click on the following menu:
Image | Adjustments | Curves (or press the shortcut key Ctrl+M)
You will see three eye-droppers in the bottom right hand corner of this dialog. If you mouse-over them, you will see the bottom left is Black, the middle is Gray and the right one is White.

Figure 1. Curves Adjustment - Setting Black, White and Gray Target Defaults

You will now be setting up the preset target defaults for your Shadow (Black), Highlight (White) and Midtone (Gray) values respectively. Generally accepted RGB (Red | Green | Blue) standards are R:20 G:20 B:20 for Shadow, R:244 G:244 B:244 for Highlight and R:133 G:133 B:133 for Midtones. These values will ensure that there is enough shadow and highlight detail in your image.

Figure 2. Select Target Shadow Colour

Double-click on the Black eye-dropper and enter the values 20, 20 and 20 in the Red, Green and Blue fields respectively as per Figure 2. Select Target Shadow Color and click OK.

Figure 3. Select Target Highlight Color

Double-click on the White eye-dropper and enter the values 244, 244, 244 respectively as per Figure 3. Select Target Highlight Color and click OK.

Figure 4. Select Target Midtone Color

Double-click on the Gray eye-dropper and enter the values 133, 133, 133 respectively as per Figure 4. Select Target Midtone Color and click OK.

Clicking OK on the Curves Adjustment dialog brings up a message-box asking if you want to save these new target colours as the default (see Figure 4) at which point you should select 'Yes'. Once this is done, these values are permanently stored as your Shadow, Highlight and Midtone values. Of course it also pre-supposes that you will be working in RGB mode most of the time, which I'm assuming you will be doing. To double-check this, you can always go to the menu: Image | Mode | RGB Color

Figure 5. Default Target Colours

There is one more final step to ensure accurate colour adjustment and that is to make sure your colour picker is setup properly.

Click on the Eyedropper Tool from the tools Palette (or press the shortcut key I). In the tool menu at the top of the screen, make sure that the Sample size is set to '3 by 3 Average'. Point sample is too narrow and 5 by 5 not really necessary. 3 by 3 will ensure that you get an average colour over a selection of 9 pixels rather than just the one pixel pointed at by the Eyedropper. This is important in colour cast conversions, particularly for skin on faces as we don't want to pick up any blemishes for and we want an overall adjusted colour.

Right, that's the setup out of the way.

Colour Correction the Easy Way - Using A Photoshop Action

Now let's move on to the more interesting part of this tutorial and that is how to colour correct our images. As mentioned I am providing a Photoshop Action which you can download here:

Color Correction Action

To save this file, right-click on the hyperlink above and select 'Save Link As' (if you're using FireFox) or 'Save Target As' (if you're using Internet Explorer). Save the Action file to the default directory for your Photoshop Actions (this is usually C:\Program Files\Adobe\Adobe Photoshop CS2\Presets\Photoshop Actions but may be different depending on your installation).
When the .ZIP file has downloaded extract it into this same folder.

To load the Action into Photoshop, make sure the Action's palette is visible, using either the menu item: Windows | Action (or the shortcut key Alt+F9)

Now click on the tiny arrow on the right of the window and choose the Load Actions menu item that pops-up. Click on the ColorCorrection.atn file that should be in the folder listed (by default Photoshop goes to the folder mentioned above, if yours is different then you will need to navigate to the folder where you saved the file).

The Action now appears in the palette ready for action (so to speak). see Figure 6. Actions Palette; yours may look slightly different. A couple of pointers as an aside, you will notice that this action has a red dialog next to it. This means that this particular action requires some user-interaction and that is the next part that we are going to cover. It's fairly straightforward if you follow these steps as outlined below exactly. Doing it differently and you may introduce colour casts to your image instead of removing them.

Figure 6. Actions Palette

Still in the Actions Palette, click on the 'Color Correction' item (not the folder icon). When this is highlighted (and you have an image loading in Photoshop that requires colour correcting of course!), click on the Play button.

The first screen that pops-up is a Threshold Layer dialog (see Figure 7. Threshold Layer). Don't worry too much about this now, just click on the OK button to dismiss it.

Figure 7. Threshold Layer Dialog

Once that dialog has been dismissed Photoshop will do something weird to your image and pop-up a Threshold Levels dialog (see Figure 8. Threshold Levels - Set Shadow Point). This is where the fun starts! (whew finally, I never thought he was going to get there - Ed!)

We're going to set up our Shadow (Black) point first, so start by moving the little white slider (underneath the graph/histogram) ALL the way to the LEFT hand side.
Now you want to slowly move that slider (the Threshold Level indicator) slowly back to the RIGHT until you begin to see some BLACK areas appearing in your image. These are the areas that are true black (your shadow points).
Find a suitable area on your picture that is showing up in black and using the Eye-Dropper tool (I) Shift+Click on that area of the image. This sets up point #1 which is your absolute Shadow value.

Figure 8. Threshold Levels - Set Shadow Point

Now you need to move the all the way to the RIGHT hand side of the histogram and slowly move it back to the LEFT until you see some areas of WHITE begin to appear in your image. These are the areas of absolute White (your highlights). Once you have identified a suitable location using the Eye-Dropper tool again, Shift+Click on that location (note, it may be useful to Zoom in on the image to make an accurate point selection). This sets up point #2 which is your absolute Highlight value. (see Figure 9. Threshold Levels - Set Highlight Point)

Figure 9. Threshold Levels - Set Highlight Point

So now we have on our image two colour sampling points which represent our absolute shadow and our absolute highlight. Clicking OK on this dialog takes us to the next step and this is to find a perfect midtone (that 50% gray) in our image where we can mark our next point.

The Photoshop action now removes that previous Threshold Layer and creates a new overlay (which is overlaid in difference mode with an 50% gray mask - more detail is provided in the advanced section below). Your image will look a bit strange in the background and another Threshold Layers dialog appears (identical to Figure 7. Threshold Layer Dialog). Simply click OK for now as this is a temporary layer anyway.

A new Threshold Levels Histogram appears and this is our Mid-tone Histogram (see Figure 10. Threshold Levels - Set Midtone Point). In an identical fashion as before move the level slider all the way to the LEFT of the histogram and then slowly move it RIGHT until BLACK areas start to appear on your WHITE background. Those black areas represent the gray midtone markers on your image. This is a fantastic way of finding your mid-tone values and removes ALL of the guess-work out of it that you may have employed before trying to find that perfect mid-tone.
When you have a suitable region Shift+Click using the Eye-Dropper tool to create point #3 which is your mid-tone value.

Figure 10. Threshold Levels - Set Midtone Point

Click OK on this dialog as we are done with it. The Action now creates a new Layer which will become your Curves adjustment to perform the Colour Correction. Simply click OK on this dialog (see Figure 11. Colour Correction Layer)

Figure 11. Colour Correction Layer

At this point you are presented with the typical Curves Adjustment dialog (as per Figure 1. Curves Adjustment - Setting Black, Gray and White Target Defaults). We are now going to use our defined points to setup our Shadow (#1), Highlight (#2) and Mid-Tone (#3) values.

(As as aside you may notice that I have more grid-lines than is the usual default provided in Photoshop. To do this simply Alt+Click on the grid. Having more grid lines provides greater flexibility in setting up more accurate curves for tonal adjustments).

Click on the 'Set Black Point' eye-dropper (this is the left-one, mouse-over it to be sure you are selecting the right one). While it is selected move the mouse to your image and click directly in the middle of the Point #1 that you created earlier. This has set your Shadow threshold.

Click on the 'Set White Point' eye-dropper (this is the right-one) and click directly in the middle of the Point #2 that you created earlier. You have now set your Highlight threshold.

Finally click on the 'Set Gray Point' eye-dropper and click directly in the middle of the Point #3 created earlier. You have now set your Midtone threshold value.

With the 'Preview' check-box enabled you will immediately be able to see the changes this makes to your image and should automatically remove any colour-cast that you may have on your image.

Finally Click OK on the Curves Adjustment dialog. The Photoshop Action renames the layer to 'Color Correction' and removes the three colour sampling points marked out earlier. Clicking the enabled icon (the small eye) next to the 'Color Correction' layer allows you to see a before and after image of the Colour Cast changes. The beauty of all this now is that the Colour Correction has been applied as a Curves Adjustment layer and you can manipulate the curve as you see fit to make the image more suitable and pleasing to the eye if required. If you are not used to using Curves however, this Action has done all the work for you and you haven't moved a single Curve.

In a nut-shell that is a simple way to fix colour-casts. It takes a bit of explaining in words but with a little bit of practice it takes less than a minute to apply this action and set the points accordingly. When should you use it? Well I personally make it my first step in my Workflow to make sure that I am working with a cast free image. This is particularly important I have found when working with an Adobe RGB image that is going to be converted to sRGB for web or email distribution as that conversion typically reveals the colour-cast quite dramatically.

The rest of this tutorial is devoted to an explanation of what is going on behind the scenes in the Action and is not required reading.

Good luck.

Behind the Scenes - Color Correction Action

The first thing this action does is to flatten the current image so it's always a good idea to have saved your current layer set to a .PSD file in case you wish to revert to it. I may enhance this action to create an Undo point but that is for the future.

We then utilise a standard Theshold Layer to determine the absolute Shadow and Highlight points. Remember we set our Shadow levels to 20:20:20 and our Highlights to 244:244:244 so these are not absolutes but provide an evenly balanced image which is more suitable for print output of some nature.

This adjustment layer prompts for user input in the action and it is there that we can capture Points #1 and Points #2 for the Shadow and Highlight levels.

This is all fairly straight-forward so far. The really interesting bit comes in creating a Difference Layer with a 50% Gray mask which allows us to find that mid-tone point, point #3. The Action achieves this by creating a new layer, filling it with a 50% gray colour and blends it with your original image in Difference mode. The Threshold level adjustment then allows us to find that perfect gray colour in our image so simply. This works almost all of the time, but be aware that you will find photo's that just don't have neutral midtone so you might just have to go back to guessing?

Finally the action cleans up after itself and leaves you with a Curves Adjustment layer.

I hope that you have found this tutorial interesting and informative. Feel free to send me feedback at ian.weatherburn@icon.co.za
My photography can be viewed at http://www.ianweatherburn.com (click on Gallery)