This is a bad idea, I gunsmith on an apprentice basis, and the rapid heating and cooling of the slide could possibly alter the metallurgy of the slide, even small amounts of heat that is not uniform could warp the action,(especially when rapidly cooling it in a freezer) if you want to add some color to your gun i recommend chalk, i have used that several times to highlight iron sights and it could work in this application without the heat, just rub it across the grooves and it will stick. i guess you could also use nail polish too, Ive seen that used to highlight iron sights also ,just make sure you wipe off the excess before it dries.