If the suit was left wet overnight in a damp towel it is possible that non fast dye in the towel have migrated to the speedos. The dye may have then become fast in the synthetic Lycra fabric.
Although one should never machine wash Lycra with synthetic detergents and bleaches you may be past the point of caring.
In the UK we have a fabric stain remover called Vanish, I didn't realise they have an Oxi Action non bleaching product for whites. It's expensive but if your speedos are much loved you could try it. Please let me know if it works. Video your efforts and send them to Vanish and you could get into a commercial.
https://www.amazon.co.uk/Vanish-Fabr...NsaWNrPXRydWU=
Product information:
https://www.vanish.co.uk/videos/how-...es-whiter/?p=2