If you are baking a butter cake, it will only stay perfectly moist the day it is baked, at most one day later if you have wrapped it really well in plastic wrap and then foil as soon as it is cool. If you want to bake it well ahead, you’ll need to syrup it according to the directions in the wedding cake section (page number in the 500s).
Each recipe has guidelines for storage in the sidebar that will tell you how long you can store the cake. I would definitely keep it in the fridge if you have room, mousseline will be fine refrigerated or frozen, just don’t refrigerate fondant.
Mousseline is off-white, a result of the mixture of pale yellow butter and white meringue. It is the whitest of the buttercreams. I think beating the butter until lightened in color and flufffy before incorporating it into the meringue gives the palest ivory (as opposed to using it straight from the stick).