kiwigal81 is correct. You can add vodka or vanilla extract to the powdered or even gel food color and write using a very fine brush (0 or 00). You only juse a little alcohol with the color. Make sure you practice before you write on the cake!
You can buy the powdered or gel colors from Global Sugar Art which has wonderful online service and a great customer service department! THey also ship world wide!!
I dont use vodka or everclear because I dont like the smell of the alcohol. But lemon extract or a clear vanilla will work. If you use a regular vanilla it wont give you the true color due to the tint of the extract.
Mix small amounts of the extract at a time because its easier to come up with the right consistancy than if you try vise/versa.
They also sell whats called a waterpen that you can fill your color into and it will give you better control of the flow of the color. If you need something very fine writing I use a toothpick or they have have what looks like an old fashion pen and ink (back with the feathers! and dipping them in bottles of ink!!) and you can do the same thing. Lots of little gadgets you can buy to get clean lettering, etc. Hope this helps!
Airbrush colors are nice to paint with too. They are already thinned down. 
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