I know that most of "why" questions don't make much sense as far as linguistics are concerned but I'll ask anyway. I know that き means a tree. いろ means color. It doesn't take a genius to guess that ...
赤 is generally understood as meaning 'red' (which can explain combinations like 赤子 (red face of a crying baby)). However, I have seen some words with 赤 that have a negative meaning, 赤 possibly ...