definitely build your color put in a little yellow, mix it in, then some red, then add your brown and give it a good mix and it should come to a solid color brown.
On the fondant or in the fondant? If in, add cocoa powder. If on, trial and error with mixing colors to get the shade you want. Are you using Americolor? They have a nice chocolate brown and a warm brown.
i don't know just how much you need but if you'll mix 3 lbs of white fondant with rainforest green powder (roll until mixed well) then add 1 lb of chocolate fondant. the color is a beautiful velvet brown. this came from a.c.d.m.
I don't know HOW brown you need, but Satin Ice makes a chocolate fondant-it's VERY dark, (and VERY yummy), if you're not trying for a dark brown you could mix it with your other fondant to get the shade you need, ... HTH
