Correcting Unwanted Color Casts in Post Processing

Dec/10

By Allan Peterson

Have you ever seen your images awash in a hue of yellow, green or blue which didn’t seem to be there when you took the shot? Light sources give off various cool or warm color hues and if your camera’s white balance feature makes an incorrect adjustment to neutralize these colors, then they can be very visible in your shots. This tutorial will show you how to correct unwanted color casts in the post processing stage.

Keep in mind that this method is not the only way to fix images with color casts, and as with all photo editing techniques, it will be effective for some images but be less successful with others depending on the nature of the color cast.

The photo we will be using is that of Mary, the Chihuahua. Now Mary is warming herself in the morning sunlight and there is an obvious yellow cast to this shot, most evident in the areas of her fur that should be white.

This is the original shot and by the end of this tutorial, you can see a side by side comparison after we get rid of the yellow color cast. . The photo editing program used is Photoshop CS2.

Step 1:

Open your image in Photoshop and create a duplicate layer. Next, go to Image > Adjustments > Threshold and a dialog box will appear showing the threshold level. Move the slider all the way to the left until a little patch of black is left. This patch signifies that it is the blackest section in the image. Since this photo has a lot of black areas, there are big black patches even though the slider has been moved all the way to the left of the graph.

Step 2:

Hover your cursor over a black patch, which has now turned into an eyedropper tool, press and hold the Shift key and select a black patch with the eyedropper. A marker will appear with the number 1. It would look like this:

Step 3:

Next, move the threshold slider tool all the way to the right and the image preview will now show you the areas that are the lightest or whitest in color. Pick a white patch that is around 7 pixels wide, and again press and hold the Shift key then mark the white spot with the eyedropper. The new marker will be numbered ‘2’.

Press Cancel, not OK, when you’re done placing these two markers.

Step 4:

We’re going to open a different dialog box now. Go to Image > Adjustments > Curves to tweak some of the Curves settings. It’s ideal to set the white point first. Double click the white eyedropper tool on the right and the color picker box will pop up. Change the RGB values to 240 to avoid over whitening.

Next, click (just once) on the black eyedropper tool on the left and select the first marker you placed when you were setting the threshold. Then click the white eyedropper tool and select the second marker. The middle eyedropper tool sets the gray point. Choose this last and select a medium gray color in the image. If you’re not sure of your choice, double click on the eyedropper to pull up the color picker box. While keeping the left mouse button pressed, move the eyedropper tool around the image while watching the RGB values. The goal is to look for values that are equal to or as near to each other as possible since the same RGB values equal gray. When you’ve made your selection, press OK.

Step 5:

Here’s a screenshot of the eyedropper tools and the corresponding markers selected. For the gray point, the area chosen was the gray spot above Mary’s eye. When you’re all finished, press OK.

This is a comparison between the original photo and after the yellow color cast disappeared. That’s a pretty big change in less than 4 minutes’ work, right? Now the areas of Mary's fur that should be white have been restored. After practicing these steps a few times, you can remove unwanted color casts in no time.

This entry was posted on Wednesday, December 8th, 2010 at 8:41 am and is filed under Advanced Tutorials, Articles, Photography Tutorials, Photoshop Tutorials. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.