Your eye is less sensitive to blue light, which is why the OEM's use 4300k. The cheap lamps are always blue because the salts required to make a 4300k HID lamp are expensive.
While the car may pass an MOT, what actually matters is the Construction and Use regulations. A vehicle can pass the MOT, but be unsafe or illegal according to the C&U regs, because the MOT is essentially just a basic safety check. HID's are not allowed under the C&U regs AT ALL. However European type approval does allow HID's to be used subject to certain criteria being met, and those type approval regulations override the C&U regs.
So, if you want to use HID's you need to conform with the Type Approval regs which clearly state the car must have washing and levelling fitted. Theres an exemption to the levelling on a car with very stiff suspension (say a Pagani Zonda or something) where large changes in height arent likely, or vehicles with self levelling suspension. When you consider the differences between HID and halogen lighting, you can see why these items are requirements.
Most folk will take the view that the MOT is the only time their car gets looked at by anyone in authority, and if it passes that, then its fine. Ofcourse while rare, VOSA checks do happen from time to time, and should you be in a serious accident, it may be noticed that your car has illegal lighting and perhaps these were to blame (especially if theyre cheap crappy blue ebay kits!)... Ofcourse, its upto you to decide what you want to do, so long as you know all the information and can make a proper informed decision.
