I once had a student in class ask about the colors I was using on the whiteboard because he was color blind. Since then, I have tried to be good about what colors I use when teaching. Yesterday I was considering a low-tech alternative to clickers for the classroom: color cards! However, now I have this color problem again. I am too lazy to write the color of the card on the color cards I give to the class. Surreptitiously, my partner was working on a figure for a new paper and referring to the chart below. It provides a handy guideline for what colors look like to the colorblind and allows you to pick a subset of contrasting colors. For more information, see the original research by Okabe and Ito.