What?! I've NEVER seen a cake recipe that calls for soy-anything. A basic cake is butter, sugar, egg, flour, salt, vanilla and baking powder. Where are you looking at recipes?
How did it turn out in the end?
---------- Post added at 11:45 PM ---------- Previous post was at 11:41 PM ----------
They say frostings that use shortning in place of some of the butter tend to hold up better to being left out. I've never tried it personally, though because it sounds icky.
Cream cheese frosting is pretty much foolproof and would probably accept a runnier colouring than an all-butter frosting. It's stiff enough to use for piping. Do you have the right tip for fur?