If you want to go for a medium/natural brown, I suggest using an Asian brand called Palty -- it's sold only in Asian markets (your local Chinatown probably has it) but I've used it once and it's pretty effective. I dye my hair VERY often (like once every month/2 months) so I don't have many consistent pictures between my virgin black hair (many years ago) and the color I have now (which is ash dark brown/black). My hair pretty much went through the entire spectrum between black and medium light brown lol.
Here's a picture of the lightest my hair's ever been (Feb 2010):
My hair now: