help with hair dye

I want to go from a light brown to a dark dark brown / black colour over the period of a week or two. I don`t want it to from the light to the dark overnight. I want a smooth transition. Any suggestions for specific dye colours and methods of doing this?

Also, I play a lot of sports and I really mess up my hair, and it`s completely untameable. Anything that you can suggest that`s good on-the-fly, like at school?

Thanks in advance.