I'm assuming you use the red food coloring for sketching when on leatherhard clay. What about on bisque pots?
I use a straightforward HB pencil to mark out on bisque (and usually wish i hadn't - my artistic skills leave a lot to be desired) but the pencil works fine.
Oh Ayjay, that gave me a good laugh. How many times have I decided to go all Picasso on a pot and had regret... more than I'd like to admit!