Do not do it.
I think that spraying your two tone gun which is an IF (Imitation Firearm) to make it a RIF (Reailstic Imitation Firearm) is against the VCRA without a defence.
36. Manufacture, 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;
Hence people over 18 can buy an IF without needing a valid defence (site membership or UKARA) but are not permitted to buy a RIF.
You can take you gun to a site, wrap camo tape around it as a temporary change, then remove the tape at end of day so that the gun becomes an IF again.