Some very useful charts for mixing fondant colors and fondant yields per tier size have been created by Albert Uster Imports. See the attachment for both.
I received this yesterday, too, but be careful. It applies ONLY to the new Uster FondArt colors.
Those proportions may not yield the same results with any other fondant because every fondant maker has proprietary color mixes.
Also, Uster recommends that FondArt be rolled to 1/8" thick--thinner than many people go, especially without a fondant sheeter--so the most people would need more to cover the cakes, as shown.