Does anyone know what combinations of the Americolor gel I need to use to get both fondant and buttercream a champagne color?  

AFor champagne color I use Wilton Ivory. Just keep adding a little at a time until you get the color shade you want.

Okay ... so what do you use when you want ivory?  I use the ivory color (Americolor) for ivory.  So for champagne would I just add a little more?  

I would also use ivory, and maybe a little yellow too. Adding just more ivory would get you to a caramel colour if you're not careful I think. If you Google "Pantone Champagne" and look at the images, lots of different shades will come up. There is no right way of mixing a colour...you kind of have to play around with it, according to the exact colour you are looking for or trying to match.