I made a very basic coffee sponge traybake with a modest coating of coffee icing - not even buttercream, just water, coffee and icing sugar.
Cut it into squares, shoved it in a box and left it on the cool side of the kitchen, forgetting that by the end of the day the cool side of the kitchen is actually in blaring sunshine and red hot.
None of it melted as it wasn't buttercream, but all the icing caught in the sunlight has bleached to an anaemic looking grey. It tastes fine - not much of coffee I suppose, but still OK.
Presumably it's just cosmetic and it's fine to eat, other than looking a bit unappealing? I made it with decaffeinated coffee, for my sins (the heartburn is enough to keep people awake, let alone caffeine as well)...