Dean is absolutely correct here. The higher the frequency, the less visible light in the beam (and the less our brains are capable of resolving what we see cast in the light). I know blue/purple are the cool colors to have, but they're really not incredibly useful.
Your best bet is to go yellow (3-4k) if you want maximum function. Short of that, 4.5k or 5k is going to give you a nice, bright, white light. I had 55w 6000k lights in my XB12Ss, and they were too blue for my tastes, and almost too bright for night time use behind other cars (I was blinding people all the time, not necessarily improving my visibility which is the end goal). Now, with my KTM, I have 55w 3500k low beam and 35w 5000k high beam, and I'm perfectly content.