What Causes Dark Brown/purple Color Skin Patches?

What causes dark brown/purple-ish skin patches? I have 4. 3 are small, about the size of a quarter and one is about the size of a baseball. All are on my legs. 2 on one leg, 2 on the other. They do not go away. I have had them for a few years now. I recently developed a new one though. What could they be? They also itch and turn red after sun exposure. I am a caucasian female but my skin coloring is olive.