I don't remember it being an option when those machines were first being trotted out. At the time, essentially, either you got a 16mb Rage 128, or you got a 16mb Rage 128, and there wasn't much in between, at all. That didn't stop ATi from selling older cards with vidcap/tvtuner along with newer/better cards as add-ons, but it wasn't available that way from Apple.

It also may have been something someone added to a previous machine then transitioned up to the b/w when they got that, either to get the video capture functionality or to get a different display connector, as it seems like it worked fine and in OS 8.5-and-then-9, framebuffer performance was honestly not as big a deal as it is these days with OS X.