how do I make green out of brown?
sorry not sure what you mean, what do you want to make green?
You can't make green out of brown.
Green is made out of yellow and blue, or just use a green food colouring. If you want a bright, lime green then use lots of yellow and just a little green or tiny smidge of blue.
You can add a little bit of brown to green for a duller/earthier shade of green.
Once you have brown, you don't have alot of options other than adding whitener or more icing/fondant or whatever white colored product you started with in order to lighten it or you can turn it into black. As Coral3 stated, adding it to other colors for duller shades, but even then, it doesn't take much.