How does someone find out they're colour blind?

I was thinking about a story I remember one of my teachers told me when I was in primary school, the other day. He said that the only reason why the kid found out he was colour blind was because he got the colour of a texta or something like that wrong.

If someone's born colour-blind and didn't know, wouldn't you just be taught that yellow looks like a different hue of purple or something along those lines. I'd also imagine if you suddenly stopped being able to see as many colours, you'd notice; even if it was gradually over time.