OK, run a thought experiment with me.
Let's say that you're stopped on the way to or from an airsoft skirmish with that two-tone imitation firearm. What is your
reasonable excuse (the proof whereof lies with you) for having with you an imitation firearm ?
Now let's say that you paint it. Then you're stopped on the way to or from an airsoft skirmish with your realistic imitation firearm.
First, answer the exact same question as above. Then, on the vanishingly small chance that the constable detects that it used to be a two tone,
and knows that it's
an offence to modify it,
and you do something insane like volunteer the information that you modified it, what is your defence to that offence? Is it, perchance, the exact same defence as the reasonable excuse that you already need in order to be in possession of it in public?
I believe that it is. There's no minimum age limit on that defence. You don't need a "licence" to do it. In fact, no licence exists to do so.
Opinions will differ, but my position is that if the question of turning an IF into a RIF ever becomes an issue for you, then it's because you've already messed up and are in bigger trouble for simple possession.
All that said, I will parrot the party line: <SQUAWK> Wait until you're 18. <SQUAWK> Get your UKARA. <SQUAWK"> Look both ways before crossing the road.
... or, camo tape it while on site, then remove the tape for transport.
, just need to wait 4 more weeks to qualify for my "Licence". 
