Initial Teaching Alphabet perhaps? - a bit more radical than what you are describing though.
https://en.wikipedia.org/wiki/Initial_Teaching_Alphabet
As the 'pedia page says, the main issue was transfer to mainstream letters. I came through infant school a year or two after this idea was abandoned in the UK. We did have the colour coded reading books mentioned though.
No, that's not it.
I suspect that it was a personal project of some teacher at some school I was attending, or maybe it's something which I came across while studying typography which was never actually implemented.
Anyway, I think it's an idea which someone should give a try --- maybe I will some day in a future font design.