Most of the time generally when we see wood it appears to be brown due to the photons absorbed by them which are brown in color. While the color of wood is determined by its chemical makeup and its quantum mechanical interaction with light(photons). In fact it is made up of carbon chains with hydrogen and some other elements like nitrogen, oxygen, and trace minerals. So when there is a war going on between trees and their parasites, including fungi, bacteria, isects etc. Therefore the trees rely mostly on chemical warfare, by lacing their cells with chemicals that deter the parasites and some of these chemicals are of neutral color and the resulting wood appears pale and colorless, but some are of striking color and this is what produces the colorful streaks that follow the grain.