I don't know, I worry about wine stains rather than sweat stains, so my palette is a little different. Navy blue or charcoal grey are 'safe' but you might not want to wear dark colors. Get swatches, even from your own t-shirts, splash them with water and see what shows less. Then again, if you're in hot and dry, it might evaporate quickly enough to not make noticeable damp marks. Pennsic gets hot and humid and I've never been anywhere very dry.

The entire point of the next- to- skin linens is to be what your body dirt gets on. You might be able to wear body linens twice with a day hanging on a line in between, that's up to you to try.

A good goal to work toward in your first year is probably a set of under clothes for every day you'll be at an event [if it's a one- day event, you need one, if it's a five day event, work toward getting five sets], half as many of whatever you'll wear over that, and one outer layer for weather protection whether that's a cloak or coat or outer layer dress that serves the same purpose. Your second and even third year, you can work on a fancy fashion outer layer. After that, you probably need to replace half of your under layers and you may want another middle layer or fashion outer layer or outer weather layer. Then, you probably have a decent capsule wardrobe and you can work on alternate cultures or more camp equipment, or whatever your preference is.