Correcting a colour cast on digital photographs

using layer blending modes in Photoshop

All you need to make this method work is to be able to identify something in the photo that should be white.

Open the photograph in Photoshop.
Zoom in (Z) onto something that should be white (in this example, the magazines on the table in the centre of the picture)
Select the eyedropper tool (I) and left click on the photograph on something that should be white (but isn't, because of the colour cast). You only have to find one pixel! Or you can set the Sample Size to average the colour over a small area. The colour in this case was RGB<185, 176, 109>
Create a new layer over the photograph. Fill the new layer with the colour you picked up, using the paintbucket tool (G)
Invert the colour of the new layer (Ctrl I)
Change the Blending Mode of the new layer using the drop-down list in the Layers palette - select Color Dodge
When you select this, the colour cast will be corrected.
Zoom out (Ctrl 0) to see the whole photograph
Here's the original for comparison:

Variations on the technique:

• changing the transparency of the new layer varies the degree of colour correction
• you can apply a mask to the correction layer (useful, for example, if part of your image is artificial light and part of it is daylight - through a doorway or window, etc)
• you can apply a correction layer of the same colour to other photos taken in the same place which may not have anything white in them