In green plants, the main pigment is chlorophyll. The chlorophyll molecule (because of its structure) absorbs blue and red light, but not green light which is then reflected to your eye (or transmitted to your eye if you look at light through the leaf). Many algae and all cyanobacteria are other colours (pink, red, blue, brown, yellow). This is because, although they contain chlorophyll, they also contain significant levels of other pigments which absorb light at different (additional) wavelengths (colours). It is thought that they have these other pigments to maximize their energy absorption in low light conditions underwater. Brown seaweeds contain green chlorophyll and an orange pigment called fucoxanthin (also some other pigments in much smaller amounts). If you live near the sea and can get a piece of brown seaweed you can do a simple experiment. Boil it ! As the temperature rises, the colour will change from brown to green. Contrary to what you might think, the brown colour is not due to a simple mixture of chlorophyll and the fucoxanthin. In fact the chlorophyll and fucoxanthin are joined together with protein to form a very complex molecule and the brown colour is due to the changed physical stucture of the two pigments. Perhaps it is easiest to imagine it as the protein twisting the pigments and so changing the wavelengths that they absorb. When you boil it, the protein absorbs energy and unravels ("denatures") releasing the two pigments. You then see the green colour of the dominant chlorophyll, but not the orange colour of the fucoxanthin which is present in a smaller amount. Brian
Red Japanese maple and, say, Crimson Sentry maple reflect other light. Golden Hinoki Cypress reflects other light. The blue grey Manzanita, or blue fescue reflects other light. Just depends on the plant. But many reds or yellows are not natural occurring. Seems that the bluish are more likely a naturally found color.