into buttercream? or royal? Instead of brushing it on? I thought I read somewhere that one of them isn't a good idea to do that with.

Bump...because I need to find out for a cake I have to have finished by Friday!

It's certainly not a good idea to mix it into royal icing. You get a very dull icing the colour is usually incorrect and it's difficult to work with. If you want to mix colour into the buttercream I suggest you use colouring made for this purpose

I beleive I read somewhere that is a no no. It is to be brushed or painted on.