Um.. so reflective material is treated differently to lights
The Road Vehicle Lighting Regulations
https://www.legislation.gov.uk/uksi/1989/1796/made
Colour of light shown by lamps and reflectors
11.—(1) No vehicle shall be fitted with a lamp which is capable of showing a red light to the front, except–
(b)a side marker lamp or a side retro reflector;
(c)retro reflective material or a retro reflector designed primarily to reflect light to one or both sides of the vehicle and attached to or incorporated in any wheel or tyre of–
(i)a pedal cycle and any sidecar attached to it;
(2) No vehicle shall be fitted with a lamp which is capable of showing any light to the rear, other than a red light, except–
(p)reflected light from amber pedal retro reflectors;
(q)reflected light of any colour from retro reflective material or a retro reflector designed primarily to reflect light to one or both sides of the vehicle and attached to or incorporated in any wheel or tyre of–
(i)a pedal cycle and any sidecar attached to it;