Thank you, everyone, for the replies.
They tend to put something they've learned on the back burner for a while so they have space to work on learning the next new thing. .
He has become very interested in numbers recently. Constantly counting things. Perhaps his mind is too wrapped up in numbers to bother with colors right now.
Honestly, if his pediatrician is aware that your child knew his colors, or is willing to believe it, I'd ask for a referral for a complete eye exam to make sure there isn't something compromising his color vision, particularly since only the red part of the spectrum seems to be affected.
He is due for a checkup soon, so if he still seems to be struggling I will definitely discuss it with his doctor.
Did something change recently?
No changes that I can think of.
