All our swimwear turns yellow from the chemicals in the pool, in fact ruins a few every year. I left a wet swimsuit on top of white towels and it left a yellow stain. Tried every thing, cant shift it. If your guests are sitting on the bed in wet cossies, then leave it to dry, that may explain the yellow stains.
The black marks maybe waterproof makeup or henna tattoos, cant shift the tattoos, but fairy liquid rubbed on makeup seems to work sometimes.
Im not a lot of help I know, but it might suggest why the laundry service cant remove it.