On most fabrics the color you see is from the oxidized stain where some sort of bodily fluid remained in the fabric. We see it a lot on kids stuff in my Mom's consignment shop...a gal brings in her stuff that she knew she washed before putting it away, and we pull it out to inspect and it has all these dribbley spots on the front. You're all familiar with the dribble zones on babies, right?
Well, she washed, yes, and good enough for the next wear and drool session, but the stain was not fully removed, and over time/heat/exposure to the ebb and tide of household temps and humidities, that wee bit of milky dribble has magically reappeared. The same holds true for the vintage stuff. It wasn't that Peggy Sue was an uber- sweater (and in the AC free 50s, she probably was at least damp on occasion), it's just that the laundering did not remove ALL of the persp before she put the dress away.
I think the important thing is to take the measures to clean the garment, and state as such in your listing. Then, if the garment isn't "fixed" you can try the more heroic measures. If something is cotton and it has "the pits" around here it gets a bleach or a Biz treatment...and anything washable gets washed here. I dry clean darn near everything in the dry-clean only zone, and deal with what I have to afterwards.
If a dress is fun and pretty, but just bombed with issues, get it prepped now and list it on ebay in September as a Hairspray/Grease/Sock Hop/50s Party costume. I've made some good money then on otherwise unsellable items. Gals want the look, but don't want to risk "good vintage" on a night fraught with fake blood and fangs.