Good question. At each step of the instructions, there is generally a small parts list. If this parts list describes the colors, I should be able to tell that those are the pieces that need to be attached to the model in the given step. Presumably, this would be even be easier when using the “fade“ feature that grays-out the parts from previous steps.
The only time I could see this being a problem, would be if step’s parts list has identically-shaped parts with unique colors that cannot be easily distinguished in grayscale. However, this issue could probably be resolved by creating annotations with arrows indicating which color goes where.