Hi Julie I have just made a cake and painted on black sugar paste. I just mixed my gel/paste colours with some americolour white and it worked really well, in fact much more forgiving than on white. I hadn’t posted the cake on here as I think my actual painting was a bit ropey (flowers too clumped together, will think out the balance more carefully next time). I will post it on here to give you and idea of how the colours actually worked out.
Roo's Little Cake Parlour