Bake cake as directed on the box. Let the cake completely cool.
Beat together butter and shortening until smooth.
Microwave for 30 seconds on high; stir until mixed well. Continue microwaving in 30 second intervals until they are melted and smooth.
To color fondant, add a little gel color to your desired amount and knead it with your hands until it absorbs all of the color. Add more color as needed to reach the desired colored. (I use medical gloves when mixing in the color. It's very helpful while cleaning your hands after).
Follow directions above to assemble and decorate the cake.
To store the fondant, coat it with a thin layer of shortening, wrap it tightly with plastic wrap, and the place it in a resealable bag with all the air squeezed out. It will keep well in refrigerator for several weeks.
When ready to use, knead fondant until smooth. Roll out fondant 1/8 inch thick.