The red and green highlight in my attendance chart was the moment I finally got it
I set up my class attendance chart with red for absent and green for present because that is just what everyone does. One afternoon a student stayed after class and quietly asked if she could always be marked with a symbol instead, because she could not tell my red from my green at all. I ran it through a color blindness simulator and the two cells looked basically identical, which was a punch to the gut. Now I use a checkmark and an X plus different shades, and it takes me maybe ten extra seconds. If anyone has a better symbol pair that reads clearly at a glance, I would love to hear it.
Oh man this hits home. I had a parent at conferences years ago very gently tell me she couldn't read my behavior chart because the yellow and green looked the same to her, and I felt awful. I switched to a smiley and a straight face plus different patterns and honestly it works way better than I expected. Shapes plus a pattern will always beat plain colors, good on you for listening to your student.