Checking Neutral Grays
After colour balancing an image the next step is to check the Key colours to see if the mid-tones are still holding any colour cast. Checking to see if the Grays are neutral is the best one to start with as Grays are quite common in images, and they are the easiest to deal with.
If you don't know about the 'Key Colours' then read the section on Key Colour Patterns.
Reading the Image
Judging colours by eye can be quite difficult, even when your computer monitor has has been calibrated, unless the colours have some major errors. If you are working with a lot of images then you will get tired and your eyes will start to adapt to any repeated problems.
Does the image above have a colour cast?
The image has a number of Gray areas, but the one that is going to checked here is the gray shirt in the Mao portrait as this is a mid Gray.
Open the Color palette (Window > Color on the menu bar), and select the Eyedropper Tool on the Toolbar. Clicking the Eyedropper on the shirt and the colour values will be shown in the Color Palette. If this was a neutral Gray then the colour pattern should be a verticle line "|", but this is not the case in this image.
The colour pattern leans to the right with the Red being the highest. So there is a slight Red, or pink, colour cast.
Balancing the Gray Tones
This information on the Colour palette can be now be used to colour correct the image. Leave the Colour palette open (it will be needed while the colour correction is being done) and open the Curves panel.
WARNING: The sliders on the Colour palette need to be "active", meaning that they must move while the colour correction is being done as they will tell us what to do and by how much. After the Curves panel has been opened you MUST click again on the gray area being measured, in this case the gray shirt. If you don't do this the Colour sliders will remain static.
NOTE: You will NOT need to select the Eydropper Tool from the Toolbar before opening the Curves panel to do this. In all colour correction panels, Levels, Curves, etc, the mouse pointer will automatically become an Eyedropper when you move the mouse from the correction panel and move it over the image.
The mid colour on the Colour sliders is the Green, so the aim here is to lower the Red and raise the Blue so that the Colour palette shows a verticle line pattern, thus making the Gray neutral.
From the Channel drop-down list, at the top of the Curves panel, select the Red channel. Click on the middle of the curve and pull it down, watching the sliders in the Colour palette as you do so. Stop when the Red slider has come down to the Green level.
Now select the Blue channel, and this time raise the curve until the Blue slider in the Colour palette has reached the same level as the Green.
The finished image now has neutral Grays. There has not been a big change in this example as there was only a small colour cast to start with. To repeat advice given in earlier sections, if do prefer the slightly warmer colour cast then keep it if you wish.
Finally, looking at the Colour palette again and clicking on the shirt with the Eyedropper Tool you will see that the colour pattern is now verticle as it should be.
NOTE: Don't worry if the numbers in the Colour palette are not exact. It won't matter if they vary by one or two digits. Working to excat numbers will only slow you down, so just go by the pattern that the sliders form.