Tips and tricks: correcting a bad cast
by Jean-Sébastien Rousseau-Piot and Stéphane Moniotte
Do it in five steps:
1) Activate the hue/saturation dialog box (Image->adjust menu)
2) Select the channel of the bad cast, here MAGENTA
3) Click with the eyedropper somewhere inside the bad color cast in your picture
4) Lower the saturation (and in some cases the lightness). You can even adjust the color range to be modified adding or removing colors with the other eyedroppers or with the cursors which enclose the color range.
5) Click OK and it's done!