Natural Bicolour Hair

This is my belowest layer of hair.

When I was a child my hair was golden with ringlets only at the ends (I´ll try to find better pics when I´m at home in X-mas):

 

My hair gets straight and black with the time... Right now, I do not understand why the belowest layer of my hair is much lighter than the first one which is quite dark (normally my hair is down. So it cannot be because of the sun).