Do purple LEDs mix blue and red, or do they emit true violet?
99% sure they are a blue LED + red phosphor. That's "purple" LEDs. I recently ordered a few grams of the rare-earth red phosphor to play with. 

There are also relatively inexpensive and high power 405nm "UV" LEDs which can be used to cure some UV-cured materials. I have used them to cure adhesives. 

The price generally goes up as the wavelength decreases. When you get into the UVC "germicidal" category things get relatively pricey. 

Generally if they are an LED without phosphor they will specify the wavelength in nm.