What Causes Color Changes In Fingers When Exposing To Sun?

I love the sun and i usually stay out in the sun at summer times and my skin would get very dark, but during fall to early spring my complexion get really light. I guess back to its original but what I have notice this year is that my fingers at the joints stays very dark andd when I am outside and the tempreture drops my fingers get redish purple and then remainds dark. It only lightens up when I just wake up in the mornings. What is the cause of this; do you know? I am african american.