After a bit of research I have found the following about colour change ,
After the first 100 hours of operation, the salts will begin to change their color slightly as they are "broken in". By the time the 500 hour mark is reached, the color will have shifted up by as much as 250k to around 4350k. The coloring will be slightly more blue.
Color****ing will continue until the capsules no longer function. The shifting is extremely gradual and low. Even after 1500 hours, the color will only have shifter to 4600k, and to 5000k in 2500 hours. (2500 hours is equivalent to roughly 5 years of use)
0 hours = 3200 lumens (100%)
200 hours = 2880 lumens (90%)
1000 hours = 2560 lumens (80%)
1500 hours = 2400 lumens (75%)
2000 hours = 2240 lumens (70%)
So even after 4 or 5 years, a typical HID system will still be putting out double the light of a halogen bulb.
So nothing to worry about when you work out how much you use .