So for a bit more clarity, I could put bright red on top of bright green? Or any two given bright colours? Thanks for the answer by the way, helps a lot! ?
The relevant part of the legislation would be Section 36.1b
https://www.legislation.gov.uk/ukpga/2006/38/part/2/crossheading/imitation-firearms
36Manufacture, import and sale of realistic imitation firearms
(1)A person is guilty of an offence if—
(a)he manufactures a realistic imitation firearm;
(b)he modifies an imitation firearm so that it becomes a realistic imitation firearm;
(c)he sells a realistic imitation firearm; or
(d)he brings a realistic imitation firearm into Great Britain or causes one to be brought into Great Britain.
If your intent was to play airsoft then it would be potentially legal to paint it into a RIF, as your intent isn't that then you don't have a 'defence' under the act.
(Though as mentioned above, if nobody knows or cares then you don't get caught - but on a strict legal basis 'getting away with it' doesn't equal 'legal')
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However you aren't trying to change a two tone IF into a RIF, you just want to add a logo.
If you use a bright colour under the VCRA then you have not changed its status. But nobody would worry if you added the logo in black.
To remain fully VCRA IF compliant you should leave at least half of it in a bright colour, depending on how it is from new there are many 'two tones' that aren't truly VCRA IF compliant. For example a pistols upper/slide is painted the bright colour and the lower & grip are black.
IF colours are here in section 7.2:
https://www.legislation.gov.uk/uksi/2007/2606/made
(2) The colours specified in this paragraph are—
(a)bright red;
(b)bright orange;
(c)bright yellow;
(d)bright green;
(e)bright pink;
(f)bright purple; and
(g)bright blue.