Yes, it was the assignment... carbon typically comes in black. Yes, color would add a lot to differentiating bile ducts from arteries and veins. No, med students probably won't learn from this image. Yes, med students use a lot of black and white images and I haven't heard too many complaints. In fact I think it's us medical + artistic types that actually appreciate a good colored atlas.

Thanks for the feedback though, I'll think about adding some color, though as I say, the assignment was just carbon.