How to protect lace from stains when colouring a wig?

I have to colour my full cap (transparent, clear swiss lace) from medium brown to dark brown, but I want to protect the lace from getting stains. I've seen some people on youtube using different kind of products on the lace, but I don't quite get which one works best to protect the lace (without doing any other harm to the lifespan of the wig).

Some use vaseline, one had a got2be glue spray... would normal hairspray do the job too, if applying a thick layer.

Are there better products around to protect the lace from stains? Best would be things I get in any drugstore because I have to colour the wig within in the next 48 hours, thus no time for online purchase...