QUOTE I will give that a go. But will it stick and stay on rock? QUOTE
I suspect that that will depend more on the rock and it's situation than the paint.
If I mix alkanet root with water it will give no colour, if I mix it with alcohol it will still not colour, but mix it with oil and it will give a bright red, and it is a colour fast red too. If the rock is iron rich it will change the colour used to paint on it, alkanet may end up brown, similarly if copper rich then the blue will join with the red to give a dull plum.
If the rock is too smooth then an oil paint might be better..............trial and error I think; might be fun
Look up experimental archaeology, cave painting, and see what turns up.
cheers,
Toddy