> With the photoshop channel mixer you can adjust the rgb values for the
> output channels. In other words, for each channel r, g, and b, you can
> adjust the rgb mix.
This sounds a little like the Curves tool in the gimp. Right-click on the
image, it's in <image> / Image / Colors / Curves. I'm not sure if that's
quite what you need... The Curves dialog box does let you modify each of the
color channels separately, though, as well as the Value (black & white), and
Gimp-user mailing list