Like a helluva lot of the male sex, I am partially colour blind. I have little or no trouble with primary colours in natural light, but when I was a kid at school I had a lauguage teacher who was a fanatic about making the kids do "corrections" to their daily written "boo-boos" at home, and having them checked next day. One repeated mistake merited 100 lines--"corrections must be correct!" Bi-i-i-i-g problem for Boab.At home, we were still in the age of paraffin [kerosene] lamps. And my part colour blindness was accentuated in lamplight. As often as not I would open my "corrections" jotter next day in french/latin class and stare in horror at 100 lines done in GREEN INK!
Looked perfectly black to me, at home! So I was tersely ordered to do TWO hundred lines for the next session. In lamplight green or blue both showed up as black for me. I still confuse certain shades of blue and green.