If you are part of the 92% majority (that was not supposed to be a political comment), then you can see a number inside that circle. But I can't. The common description for my condition is being colorblind, although I prefer the more politically correct "color-challenged". I am not truly colorblind. I see some colors, just not as many as you see, and I get especially confused between red and green, shades of red and green, and colors containing either red or green (for example blue versus purple).
I will stop my car for the top (or left) traffic light, but I don't really think it is red. I would wear a red tie with a green-striped shirt if no one stops me, so I try not to own shirts with green. Or to wear ties. I'm never quite sure if the meat is cooked enough. You won't see me choosing colors of house paint or women's makeup shades, and I won't be disabling colored wires for the police bomb squad. So generally I have learned to live with this minor inconvenience, and the accompanying jokes, which has not impeded my career choice.
But the world of math is not as black and white as it used to be. A textbook author would never graph two curves in the same color. But if y = .5ex is in red and y = x2 is in green, I am going to be confused as to which curve is greater in 1 < x < 3. I also get confused by multi-color pie charts where some of those colors start to blend together. And if you really want to lose me, just show me one of those colored maps with ten or so different colors representing ten different levels of the amount of rainfall.
So if you are a teacher with control over such things, please be aware that some small percent of your students could be color-challenged and they may not even know it. The best thing you can do is provide labels in addition to different colors. Another possibility is to provide different kinds of patterns (see Excel's FORMAT, FILL, PATTERN STYLES). Lastly, choose among colors we are more likely to recognize, and especially avoid red AND green together.
Have you had any difficulties in math with a color-challenged math student?