I've got a button master which is in black and white and needs to be in navy blue and white.
Poking around GIMP turns up colorify (which puts a colored filter over an image) and colorize (which seems to do the same thing). Color-selecting for black produces an amazing crawling stripey pattern that doesn't settle into anything useful.
I remember seeing a couple or three web-based graphics programs, but I haven't been able to track them down.
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