Calaelen, thanks for the input, however, I think I need to clear something up.
I did not say to go to a salon. Never. I said to purchase salon products because you can CUSTOMIZE the developer to your needs.
Someone who wishes to dye their brown hair brown and cover old brass doesn't need the 30 or 40 volume developer that comes in boxed hair dye. THAT is where the brass comes from. And THAT is where store dyes vary from professional one....not the color, but the DEVELOPER and customization of it.
If you don't want to lift your color multiple levels, there is no need to use such a strong vol developer.
In the link I posted we purchased our own Wella Color Touch (for approx $20) and our own level of developer and dyed our hair at home.
Here is more information about dying your hair at home and the science behind it: http://killerstrands.blogspot.com/