Well here’s some random tests I did with Cat face, using the method suggested by @Coherent from http://www.instructables.com/id/Enhance-Your-Image-when-Reverse-Laser-Engraving-on/ (although slightly modified as I am not needing to paint on leather).
Very surprised & happy with the effect. Didn’t bother doing full engrave of the design as I want to get it just right before I bother with that. Can’t be bothered waiting for ages for it to finish (it took about 1hr for me to perform all these tests & modify files as I was going).
So I tested first on a piece of orange card (210gsm), then on a random scrap of 3mm thick natural vegetable tanned leather, and also part way through that job I paused & added a scrap of orange canvas that I had (which I’ve been meaning to test on for a bit now).
Settings were Engrave, 500mm/s, 3 pixel steps, 6mA power.