I'd also bet on a dye process along with a dye. Is the quilt in a lot of light? (Not direct sun, but a sunny room?)
I've seen very expensive dresses sold where the mfg did not tell the customer to remove buttons for dry cleaning. I think that when a new process is used, the mfg don't quite find all the exceptions. The button mfg could not have tested buttons on perchloroethylene and sold them as tested.
With embroidery floss, black seemed to be thinner due to process, and red had a problem with setting.