Like several others here, I used to believe that what I saw as, for example, purple, might be unique to me. that perhaps colors were a unique experience to each individual, and that this was responsible for the great disparity in each individual's favorite color. What I find amazing is that I read an article only a year or two ago (it's 2003 now - in case this site stays up for a while) that indicates that while the phenomenon may not be as extreme as I (we?) believed, differences in an individuals optic cones actually DO lead to differences in perceived colors.
A childhood belief about eyes

others believed things like this
- I believed that leaves on trees only became individual when you you close to them, and then I got glasses.— nola
- When I was a child, I used to believe that Catalonia was the unique part of land of the world.Anon
- I used to think that when we got to heaven there would be new colors that no one knew about yet.— Tracey
- I used to believe that the sun and the moon were the same thing...just different colors— Gatorgrl
- I used to believe that you couldn't marry people with different eye colors as you.Anon
- I used to believe that noodles that were dyed different colors such as green had been killed.— Tasha