As a Lakers fan, I am practically required by law to tell you that the accessories should be yellow/gold.

If I got a purple bike, I would probably go with a metallic (silver, but possibly gold) bottle cage or black. If you can get the exact same color bottle cage as the paint of your bike, that would work too, but if it's off by a bit it could look funny. Maybe you could buy purple touch-up paint from the manufacturer, paint a bottle cage, and match it exactly.

All of the bike accessories I've ever gotten have been black. I feel like they hide the dirt and grime well. It's also one of the reasons I opted to buy a black bike, as opposed to a lovely white bike of a similar model that was also in the running. I don't trust myself in keeping things immaculate, especially when I'm outdoors.