painting the airframe is not a good idea apart from the prospect of damaging the plastic any paint with a metal content could impede the operation of the drones antennas and compass and GPS
I'm not familiar with the rules in the UK, but in the US, you can't fly a SUAS at night, unless you have a SUAS certificate, a night waver, and obstruction lights that can be seen for 3 miles on your aircraft.
Can you show me where it says that hobbyists (part 101) can fly at night? I found this article, although it is from 2018:
Flying Aerial Drones At Night Is NOT Permitted By The FAA | Fredericksburg Aerial Drone Photography
He says that it is a "stretch" to use part 101 for drones flying at night. - part 101 addresses Moored Balloons, Kites, Amateur Rockets, Unmanned Free Balloons, and certain model aircraft.