One year ago, I bought a bright purple orchid in bloom. One year later it is blooming again, but is stark white.
What caused this and how can I get it back to purple (violet)?
You may actually have purchased a "dyed" orchid. While a very few varieties of phaleonopsis in the blue/purple family exist, they are rare, and will produce the same colored flowers each blooming season. They're also a lighter, less vibrant color than the one you're describing.
Most orchids in that color spectrum are actually white orchids which have gone through a patented process of injecting dye into the plant. It is explained here:
During the growth process, the stalk of a white phalaenopsis orchid is injected with a blue dye solution. The intervention is performed in an environment that keeps the infection risk for the plant at a minimum. The blue color is absorbed by the orchid and creates a blue flower. Subsequent flowerings will result in white blooms.
The dye is not available to the public, nor are the exact steps of the procedure. Do-it-yourself dyeing is not encouraged, and at every home gardener’s own risk.
What has been upsetting Blue Mystique and Royal Blue Phal customers is that they believed to be buying a real blue orchid. In response to controversy that has been picked up by the media, growers now use product labels to inform buyers that a white orchid has been treated in order to produce those magnificent blue flowers, and that future blooms will be white.
Whether this is the case with yours or not, I have to agree that from now on, the flowers will be white. Obviously you've kept it healthy, so I hope you can learn to like it! Of course, you can always pick up another purple one, bearing in mind that it may only be that color for one season!
If your orchid is a Phaleonopsis or moth orchid, these do come in a range of colours, including purple and white. Because of that, its quite possible for your originally purple one to bloom a different colour the following year - its just a genetic blip, or a reversion in the DNA, and there's not much you can do about it I'm afraid.