I thought that in an atom the protrons are red, the neutrons are blue, and the electrons are yellow. (I probably saw it depicted that way in a book once)
A childhood belief about chemistry

others believed things like this
- I used to believe that if you mixed red, blue, green, and yellow paint you'd get plaid!— Judith Ann
- I used to believe that an atom bomb was a dragon in the sky.— Mazzkitten
- You know the eifel65 song "Im blue da be da ba di"
I thought he said "I'm blue I'm in need of a guy"— Anna - Eifel 65 - Blue
I'm Blue (if I was green I would die...)
[I'm blue, da ba dee, da ba di.]— Matt - The song "Reverend Blue Jeans" turned out to be "Forever in Blue Jeans." Who would have believed that?— music lover
- I thought that kids' pee was yellow and adults' pee was clear.— Aretia