I'm very interested in this topic as I was playing with stamps last night on some spare cookies. I've seen Marlyn at Montreal Confections routinely using food color markers to make hers look nice and sharp. Yesterday, I had only one good stamp using painted on gel color slightly thinned with vodka. I practiced on paper first and I have to say the paper looked much better than the cookie. I also found that I could not reapply painted on color - I needed to wash the stamp between uses and I only got one sharp stamp from the first application. I was stamping on royal icing. It was a bit of a frustrating endeavor.