In short and simple, light from the headlights must be white (or part yellow, but not amber) and tail lights, red.
Loss of any more that 15% the headlight beam puts a defect notice on the table.
Rear lights need be visible up to 200 metres. Any less puts a defect on the table.
Further, a smart mouth on the side of the road will also put a defect on the table.
What's the back story?