Plants, including algae respire in darkness and photosynthesize in light. As light intensity increases, photosynthesis and the production of oxygen also increase. So different levels of light intensity effect what types of algae form and grow. There are four major groups of algae. (Cyanophyta), the blue-green algae which I guess is actually a bacteria rather than an algae, (Chlorophyta) green algae, (Phaeophyta) brown algae and (Rhodophypta) red algae. I believe the green algae which requires a higher light intensity and is more common in a reef tank is more beneficial than the brown which requires a lower light intensity, And I think the red requires even a lower light intensity. As for the purple I have no idea. Maybe part of the Cyanophyta group and might acutally be a bacteria, not sure. Ph, salinity, nutrients etc. probably have some effect on what type of algae grow also.
This is just what I have read from books and what not, still learning myself.
I have Mostly green with a little brown in my tank. Just noticed about a week ago a small dime size purple growth on a piece of live rock. John will probably reply with some better info.