XM10K color shift?

[Warnberg]

I'm looking at the tank yesterday and see that one of my XM10k's look completely different (color wise) then the rest. I think that it may be the ballast so I shut down two of the lights, let them cool, swap the bulbs and fire them back up..... same issue, the color shift is now at the other end of the tank, so it's not the ballast but the bulb. Does anyone know what causes this? Is there anything I can do short of going out a buying another bulb at 60.00 (these bulbs are less then 2 months old use and about 3 months old from purchase date)?

Thanks