If you're using the liquid food colours available in supermarkets (not concentrated) you'll probably be able to paint them strait on without diluting, but the gel/concentrated colours would need thinning and if you want to pick a particular shade then there isn't always a food colour to match.
The best way to work out how to make your paint is to first decide how you will give it colour, there are 3 main ways of doing this:
- Tint Powder
- Lustre Dust
- Food Colouring
For white paint, use Icing Whitener instead of a tint.
Then just paint on to the cake, and allow to dry.