I did something similar when we got my young dog as a pup. I went shopping for a collar with my youngest daughter, and we both agreed that we needed a RED puppy collar, as the other two dogs also had red collars and we were, as my daughter said 'a red-collar family'.
So we carefully selected a puppy collar of the right colour and size, bought it, brought it home - whereupon to our collective astonishment, we found we had bought a BLUE collar.
Neither of us is colourblind, we know, absolutely, that we discussed getting a red collar and picked up a red collar. My daughter even wondered if red collars were so popular they'd only had one left in the shop and the assistant had switched it for a blue one at the 'bagging' stage. Which seems a little far-fetched to me, but there is no other explanation as to why BOTH of us thought we'd bought a red collar, only to end up with a blue one. (She's got a red collar now. She grew out of the blue one in about a fortnight).