I'd like to second the color blindness concern. As someone with a color vision deficiency (deuteranopia, commonly known as red-green color blindness), I can't e

I'd like to second the color blindness concern. As someone with a color vision deficiency (deuteranopia, commonly known as red-green color blindness), I can't even enumerate the times I've been unable to complete some task because of a color coding which I couldn't distinguish. Worse yet, it often takes me a long time of trying to figure out how I'm supposed to know something before I suddenly stop and go "Oh, this might be color coded." In high school, whenever I felt that I suddenly couldn't follow some explanation on the whiteboard, that's when I knew the drawings were color coded. The fact that things were color coded was usually communicated implicitly. Often, this is done by drawing two sets of things in different colors, usually red and green, and then just pointing at one instance from each category while giving some explanation regarding that category. This is simple, possibly not even a conscious choice, and it works perfectly as long as the students have the slightest bit of logical reasoning skills... and can actually tell that there are multiple colors being used.