How Best to Wash a Cap?

With all the adds on here I figured it'd be great to get an expert opinion on how best to cleanse a baseball cap of sweat stains. I've tried letting it soak in detergent, however I can still see a difference in coloration. https://i.ani.me/0383/5232/1000009741.jpg Personally I love wearing lighter colored hats. It's simply frustrating not knowing what to do about getting rid of those hard to remove stains! https://i.ani.me/0383/5228/1000009740.jpg Any advice from the peanut gallery? I'm even up for an Indian bot if they know what's up!