Someone asked me what I meant by this the other month, and since I use the term so often, I figure it is worth doing as a Comic Dictionary entry.

Quoting Wikipedia's entry for colorforms, the toy colorforms are "paper-thin, die-cut vinyl sheet images and shapes that could be applied to a slick cardboard background board, much like placing paper-dolls against a paper backdrop. The images would stick to the background via static cling and could be repositioned to create new scenes."

Therefore, when I refer to a colorform comic book cover, I'm talking about a cover where it appears as though the cover artist drew a background (often by copying a photograph of a locale) and then photoshopped other figures s/he drew on to the background, making the figures not exactly appear as though they organically belong on the background, but rather, that they are like colorforms on a background.