Converting Image Colors Automagically?

[nautical9]

Hey folks, got a (hopefully) easy question, but just can't seem to figure it out myself after several hours of searching/reading.

I have a dynamic image that comes in (.gif format), and all I want to do is automatically convert certain RGB values to other colors across the entire pic (shell script, so tool must be non-interactive). I figured it'd be dirt simple, but for the life of me I can't find a tool that does it to my satisfaction.

ImageMagick was my first obvious tool of choice (mogrify specifically), but there just doesn't seem to be a simple color transform option. The best I could find was the -draw option, using the color replace, but unfortunately it has a deficiency: it only takes pixel co-ords as a parameter, so I'd have to know in advance which pixels are the target color I want to change. But in my case I don't know that, as the image changes frequently.

I've also briefly looked at several other tools (which I've already forgotten), but none of them seemed to do what I want.

Can anyone point me to something better? (or for my ultimate embarrassment, give me the proper mogrify options that I've missed? )

(ps. sorry for spamming the Gentoo forums with this, but I don't know of a better linux community on the web for general computing questions)