Spray food coloring has to be one of my favorite things to use when I'm baking. Whenever you watch Ace of Cakes or DC Cupcakes, they always use air-brush food color, and Color Mist by Wilton is the closest I've gotten to air-brushing my icing or fondant like the pros.
The picture I posted is of a cake I made for a friend's birthday. I covered the cake in a thin sheet of plain white fondant, then I used my Color Mist spray to give it a tie-dye effect.... it came out so cool! I used pink and yellow, which gave the cake a pink/orange/yellow tie-dye look. Wilton sells every color of the rainbow, and they are really inexpensive. One bottle is less then $4!
In addition to spraying fondant, I have also used my Color Mist spray on cupcakes and cakes. After piping the icing on top of a cupcake and briefly spraying it with some color, it gives it a really cool professional-looking effect where some white and some color are showing at the same time.
Using this spray makes everything look unbelievably professional... which is always what I'm striving for! It's cheap, easy to use, and will make any cake or cupcake look amazing! The only stores I've seen the spray at are specialty stores such as Michaels or A.C. Moore, but if you look here you can find your nearest retailer.
Color Mist food coloring spray is something every baker needs!