Thats news to me. I've been painting for 20 years and I've never heard that one. Maybe there is something here I'm unaware of. I appreciate it if you could get some details of what type and brand of plaster that was used.
I've always primed unpainted plaster with an alkyd "enamel underbody" primer. Once dry, it can then be painted with any kind of paint you wish.
In leu of painting, I'd try to use chlorine bleach. Try dabbing some in an inconspicous area first to see if the bleach itself will leave a stain.