I've gotten a few dark sweat spots on mine.(mine are black) But, I'd been dancing for several long sets in a very hot room. (Think Bikram Yoga Hot).
We're talking this was a really heavy workout kind of sport sweat.. There wasn't a dry place on me and it was just rolling off my face.
It was like a bad soda/mint gum commercial.

While yeah it did show, if I tossed on a dry hip wrap... You wouldn't know.

I would let her know that it has to be a very perfuse sweat before it shows and that having a dry clean hip wrap to change to once you do start sweating like that.. Would be a very good thing.

The lighter colors will show and may get that yellowish sweat stain without a dash of white vinegar or oxy clean in the wash.

The great part about the pants.... is that it did help wick away some of my sweat and dried pretty quickly.

I am "the sweaty dancer"..and it will show on the pants. I wear a hip cover every time and it covers all the marks!