Light and Dark

I always found it a bit odd that yellow is light and purple is dark. This is a long standing convention for associating colors. Obviously White and Black can be used instead of Yellow and Purple but if you want to have any depth using black and white the whole thing just becomes shades of gray. By using yellow and purple you can instead use shades of each and no matter how bright your purple or dark your yellow they can't be mixed up.

I mean you can have a bright and vibrant purple, though I guess then it becomes pink or magenta or some such. I do think of yellow as being bright but the color I think of as being really bright is green. Especially on computer screens.