VCRA Section 39, (2) ( b )
Specification for imitation firearms
A person is guilty of an offence if
he modifies an imitation firearm so that it ceases to conform to the specifications so required of it;
The question was how come you aren't allowed to spray a gun, therefore changing an imitation firearm into a realistic imitation firearm. Both are separate entities under the legislation. This is the section in law that technically prevents you doing this.
But, another question is will you be arrested for doing this and charged to court? Very, very unlikely. Not by any of the cops I know anyway, and trust me, that's a lot.