I'm going to be making a cake for my father in law with music notes.

What can I use to draw music notes on the fondant? I bought the wilton edible markers but it doesn't work very well.
I will also want to draw some music notes on some cake balls. I will be using white vanilla candy melts.

Is there some type of recipe I can make up with food coloring and use a paint brush to draw them on?

Why do you want to draw them on?
Why not just pipe them w/b'cream or royal icing?

Duff makes great color pens. I used them on both chocolate and fondant. I found them at Michaels

A script liner brush and black icing color (americolor super black) would be my method of choice. Don't mix the icing color with anything.

For the cake balls, markers are not going to work and icing color would not either. Best bet is probably to pipe the notes on with black candy melts or dark chocolate.

Americolor markers are the best! Or paint with vodka, lemon extract or vanilla and gel color mixed. I am a better painter than piper, but it depends what you are better at. You could also cut out fondant music notes with a cookie cutter. I have done that...