Well, there are different kinds of algae. Most common are green algae I suppose. There are also brown algae and black "hairy" algae (as someone has it framed on Fish Lore, lol - I liked that name, "hairy", lol). I think you can find photos of algae on PlantGeek.com, try it.
P.S. There is also a type of algae called "cyano-bacteria" that are bluish algae and which are TOXIC to fish!