Average blur to reduce colour cast

Open an image with possible colour cast

Duplicate image layer

filter-blur-average blur (this averages all colours in the image)

adjustment layer-curve on top layer

In properties of curve layer select middle picker (sample in image to select gray point) and you will see that the curves have shifted for RGB. Now delete the gray layer(background copy) and any colour cast will have been removed. This could be a useful action.