I made two batches of mod choc for the monkey, one in a pale blue and one in chocolate color. The blue was dusted with some petal dust in places to give it some additional shading when it was on the monkey.
While you can mix in powdered sugar to stiffen up the white mod choc (and if you do, just add small amounts gradually until it's stiff enough for your particular needs), but I prefer to knead in a little bit of fondant, which is what I did for the William cake.
For a light brown, I would either make a batch of white mod choc and add some chocolate fondant to it to get the shade of brown I like, or make two batches of mod choc - one white and one chocolate - and mix them to get the shade I wanted. If you or the person receiving the cake don't like milk chocolate flavors (weird, I know, but they're out there!) you can make the white and use brown icing coloring.
Chocolate should normally not be colored with regular icing colors due to seizing, but once the mod choc has been made, you can then add icing color to it and don't have to use special colorings that are meant for chocolate.
Did that help, or did I ramble? That felt sort of rambly.