When it’s on the back of your hands and arms, doctors call it "actinic purpura," "solar purpura," or "Bateman's purpura." A light knock can cause it, and it’s more common on thin, wrinkled, or sun-damaged older skin. You’re also more likely to bruise if you take drugs like aspirin or other blood thinners, or drink alcohol often. It starts as blotches of red that turn purple, then darken and fade. It doesn’t usually hurt, but it can last longer than a normal bruise, often a few weeks.