Image masking in another color?

Hi,

Does anybody know how to "mask" a colour in a picture with another color. Maybe this techique is not called masking, don't know any other term. Practically it's replacing one color with another (in the whole pic).

I'm looking for something like:
MaskImage ImageHandler, R-src, G-src, B-src, R-Dest, G-Dest, B-Dest
MaskImage MyPic,255,0,255,100,110,120

Or probably someone has written already a routine to do this? I need a really fast blitting mechanism.

Thanks in advance,

Paul