One time,in another life (during high-school) I wanted a white blouse to be pink. So I dyed the blouse, buttons and all. I also dyed a long string of plastic pearls. Viola! Everything was beautiful. I STILL have the pink pearls, and it's nearly fifty years later!

Just curious..are you sure they are plastic? I have some old buttons that I believe are ivory and they have turned yellow. I like them as they are!

I you are not successful. You might consider offering them to another crafter who can make great use of yellowed buttons. A primitive crafter or dollmaker. They are always using mismatched buttons so color is not an issue.

You can use Alka Selter tablets to clean porcelain you might try that.

I guess I would experiment with some dyes. Acid dyes work better with synthetics than Procyon dyes I think. Googgle it, or just get a litte Rit from the store and experiment. never know what you might discover.

What I would do is sell the old buttons to some collector and buy new buttons with the money. Some of the old buttons I have when I use them they crack after washing them.

wow, thanks for all the great answers.
I wish these were real pretty buttons, but they didn't turn 'equally' all over. It's not a pretty look, it's just a kind of 'dirty' look.
I have enough to try some of all of these suggestions, and it sounds like painting and using for decorative purposes may be the way to go. I hate to waste anything, lol ;)

Are they pearl buttons? Pearl buttons can't be washed in detergents. It will discolor them. It takes the shiny layer off the button.

Sweet idea....

You would likely have better luck dying rather than bleaching. Best you can hope for is pale pastels.