Happens to me too. It's well within the range of normal. I just toss underpants regularly and primarily buy only black underwear -- they still stain and bleach out a bit in time, but they don't get as stained as lighter underpants.

I don't think it necessarily sounds like incontinence. (Even if it is, a drop or two of urine here and there isn't necessarily unusual or problematic.) Many, many, MANY vaginas' normal discharge has a yellowish or off-white colour, and dries darker.