That only states that red lights are not allowed, not that all other colours are.
Another part of the regs:
The restrictions state that no vehicle, other than an emergency vehicle, shall be fitted with a "blue warning beacon or special warning lamp", or a device which resembles a blue warning beacon or a special warning lamp, whether it works or not.
So that's 2 colours not allowed. When I have more time I'll search again.
Once again you're absolutely incorrect. All other colours are allowed including blue as long as they are a solid continuous colour and not flashing hence 'warning beacon'
That's why you see trucks all over the UK showing a variety of different colours to the front as it's allowed under the current legislation.
Lamps to show a steady light
13.—(1) Save as provided in paragraph (2), no vehicle shall be fitted with a lamp which automatically emits a flashing light.
(2) Paragraph (1) does not apply in respect of–
(a)a direction indicator;
(b)a headlamp fitted to an emergency vehicle [F1or a vehicle used for special forces purposes];
(c)a warning beacon or special warning lamp;
(d)a lamp or illuminated sign fitted to a vehicle used for police purposes;
(e)a green warning lamp used as an anti-lock brake indicator; F2...
(f)lamps forming part of a traffic sign;
[F3(g)a front position lamp capable of emitting a flashing light (whether or not it is also capable of emitting a steady light) which is fitted to—
(i)a pedal cycle; or
(ii)a trailer drawn by, or a sidecar attached to, a pedal cycle;
and which, if it is a lamp which is required to be fitted pursuant to regulation 18, is capable, when emitting a flashing light, of emitting light to the front of the pedal cycle, trailer or sidecar (as the case may be) of an intensity of not less than 4 candelas; or
(h)a rear position lamp capable of emitting a flashing light (whether or not it is also capable of emitting a steady light) which is fitted to—
(i)a pedal cycle; or
(ii)a trailer drawn by, or a sidecar attached to, a pedal cycle;
and which, if it is a lamp which is required to be fitted pursuant to regulation 18, is capable, when emitting a flashing light, of emitting light to the rear of the pedal cycle, trailer or sidecar (as the case may be) of an intensity of not less than 4 candelas.]