I dont' find this to be true. I've used Guinness in soap (a very dark stout) with no real colorant, and my soap was slightly ecru. I think it has a lot to do with if you gel your soap, and the oils used. I'm one of those that don't simmer the alcohol out of my beer. I just let it sit open for a few hours to a day so it will go flat, add a pinch of salt (this will help it go flat) soap at room temp and either let it sit uncovered on the counter, or throw it in the fridge - depending on the fragrance used. My beer and milk soaps are the only soaps I try to discourage gel in because they do tend to heat. By not simmering out the alcohol my soap does get thicker faster, but it's nothing I'm not used to or not prepared for, so it's just not a problem for me.