I have travertine and colored grout (brownish) in our shower. I am starting to see mold grow at the bottom where the shower stays wet after we are done using it. Room ventilation is an issue that I can't correct right now.

What is the best choice for a spray (either store bought or homemade) that I can leave on for a bit and then rinse off to clean the mold that forms on the grout? I don't want to ruin the color / durability of the grout, but need something strong enough to deal with the black stains.

Thanks!