Hang on, do you already have it, and it's in realistic colours? Then you need to do nothing. It's not an offence to obtain or own a realistic imitation firearm. Any issues are with the seller, they're not your concern.
It is an offence under the Firearms Act to possess any imitation firearm (realistic or otherwise) in public, which is why you should have a defence to hand when you're transporting it - e.g. booking slip/receipt for an airsoft site, map, contact details for a site owner.
The colour is not relevant (legally) for that purpose, although a day-glo gun should be less likely to result in questions in the very unlikely event that you're stopped. If you do decide to paint it for peace of mind, I'd suggest looking into using Plasti-Dip which will come off easily when you're comfortable with using it as a RIF.
Be sensible and it shouldn't be a problem for you to use it as-is though.