Taken from the Automotive Lighting FAQ:
What is color shift?
The labled light color on an HID bulb is an average that it will maintain over 2-3000 hours. After passing around 100-500 hours the phenonmen "color shift" will take place. Color will change slightly from a yellowish tone to a crisper bluer tone. The magnitude of color shift will vary depending on brand, model and rated color. It is a very gradual change and unless paid close attention to, a customer might not notice. OEM 4100K Bulbs manufactured by Philips has been reported to have a more noticeable color shift than OEM 4100K Osram. OEM 4100K Bulbs manufactured by Philips has a colorshift of approx 250K after 500 hours, which will bring it to up to 4350K.