Women only! Serious personal woman issue?!

The stains are from vaginal discharge, not from urine.

A lot of women refuse to buy light colored underwear for this reason.

You can either buy all white underwear and use bleach or OxyClean in your wash (rinse twice), use whatever color underwear you want and pretreat the stains before washing with a spray-on stain remover, or buy dark colored underwear, and not worry about it, since the stains won't show.

Buying dark colored underwear seems to be the most popular choice. If you buy black, it also hides period stains, and can be sexy. Although, you'll probably still want a couple of pairs of light colored underwear to wear under thin, light colored pants.

If you notice a strange color, then you might want to check for a yeast infection, but otherwise you should be experiencing just normal discharge. As for the stains, try to bleach them out or wear different underwear. I don't think that a little bit of discoloration on the inside of your panties is a big deal, nobody will notice unless they're staring right at the stain. I would probably just throw them out if it gets dark. A pale yellow shadow won't be visible to anyone other than yourself. As long as you're clean and your laundry is too, then don't worry about it.