Someone sent me a photo of a buttercream wedding cake that is all white with a white bead border & some medium sized silver dragees on it (looks like maybe 8mm). She said her colors are purple & yellow & that she wants those colors somewhere on the cake. From what I see, the only option is either to do the borders in those colors or the dragees because that's all that is on the cake. My question is... do purple & yellow dragees even exist? What about edible pearls in those colors & size? Where can I get these?

You can color edible pearls by rolling them in petal dusts.

Pre-colored ones also exist in different sizes at craft stores. 

Sixlets makes different colors, too, and seasonal colors are available at craft stores.  Yellow and purple at Easter time.  Purple at Halloween, as well.   I find them at GFS (US...not sure where you are located) too.

I think you can buy them bulk, too, online.  Not 100% on that though.

Globalsugarart.com has almost every size and color of Dragees. I have definitely seen purple, and im pretty sure yellow!