The bedding I use is white with this lovely pastel pink China vase inspired pattern. Recently, I've noticed the pillowcases look more yellowed than the duvet cover, causing the white/pastel pink to become rather dull, especially when compared against the duvet, where the white is still white and so the pattern still looks crisp.
I know why this has happened, but does anyone know how to fix this? I can see lots of advice online for plain white pillowcases, but none for patterned ones, and I don't want to risk damaging the pillowcases as the set is no longer sold. It's been like this for a while and I'm worried they're a lost cause now. 
Thank you in advance!