I have a white front-facing light and a red light at the back. However, I was wondering if I could add the blue light at the front as well from a legal standpoint. Whether it is flashing or constant doesn't matter to me, I was just wondering if it was okay to have another type of colour at the front to get noticed, and this is the only other colour I have available.
Restrictions on fitting blue warning beacons, special warning lamps and similar devices
No vehicle, other than an emergency vehicle, shall be fitted with–
(a) a blue warning beacon or special warning lamp, or
(b) a device which resembles a blue warning beacon or a special warning lamp, whether the same is in working order or not.
The Regulations have been updated multiple times since 1989, but Regulation 16 has not changed.
A warning beacon is defined by the Regulation to be "A lamp that is capable of emitting a flashing or rotating beam of light throughout 360° in the horizontal plane" and a special warning lamp is "A lamp, fitted to the front or rear of a vehicle, capable of emitting a blue flashing light and not any other kind of light". A blue flashing lamp would surely "resemble... a special warning lamp"; it's unclear whether a solid blue lamp does, but please just don't. Blue lights on emergency vehicles are useful mostly because blue uniquely means "emergency vehicle." People who see a blue light will be distracted trying to figure out if it is an emergency vehicle.
Stick to white lights on the front and red ones on the back. Note that the human eye is not very sensitive to blue (only about 20% as sensitive as to red and green), so blue is a poor choice anyway. See also Should unconventional colors be avoided for lighting?
I will speak from a personal experience. Once I had green neon lights installed under my car, they were turned on. Police stopped and told me this;
"You cannot install any kind of lights on your vehicles at any time, working or not, that would give an impression of police, ambulance, fire-brigade or doctor."
I didnt know so they let me go with a strict warning.
As long as it doesn't flash it's fine provided it isn't visible from the rear and doesn't revolve. This is presuming you are a cyclist. Wouldn't recommend for car drivers as it will be too confusing for other people and police will be pissed off.
If you are a cyclist then blue arm lights work well to signal that you are a person on a bicycle rather than a car light in the distance.