What causes our hair and eyes to change color as we age?

I would have thought that they would get lighter as we age, but mine have gotten progressively darker. I was born with very blonde hair -almost white, really. As a child it was golden blonde, eventually in my teens it became dishwater, and now at 28 I’d say my natural color is a light/medium brown.
My eyes have also gotten darker. They were clearly blue when I was a baby and a child, and as I get older they are becoming more and more green.

What actually causes this?