How to quickly remove a color cast
(without introducing a new one)

 
A color cast caused by artificial light can be difficult to remove, because simply using the grey eyedropper in 'Levels' or 'Curves' will often create a new cast in places that were not lit by the artificial light source. Scenes with mixed lighting, such as fluorescent and day light, can also be tricky. The following alternative method often works better than the grey eyedropper, and is just as easy.

Step 1:
Select the eyedropper tool and click on a place in the image that should be neutral. This is to sample the color of the color cast.
Step 2:
Make a new layer and fill this layer with the color you've just sampled (Edit - Fill - Foreground Color).

Then use 'Image - Adjustments - Invert' to invert this color. In this example the sampled color is yellow, so the inverted result is blue.

 
Step 3:
Go to the Layers pallette, and change the layer mode from 'Normal' to 'Color'. Next, change the opacity of this layer to anything that makes the original photo come through without a color cast. Usually this will be around 35%, but it can also be as low as 25% before you are satisfied with the colors.

 
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