I'm quite new to painting cookies and I use vodka to thin the gel paste in the same way I use vodka with luster dust. I found it very helpful to view Arty McGoo youtubes as well as SweetAmbs. I have also experimented with the Crystal Colors powdered food color (hydrated with vodka). I used them with a stencil and paint brush - very vivid color resulted.
Interested to try this! I used water with my first cookie painting, and I think vodka might be my preference!