I'd be very careful about offering such a service if I were you, since that will be regarded as manufacturing a Realistic Imitation Firearm as far as the law is concerned. You make that a business and you'd better be sure you have all bases covered as far as legal protection is concerned. Making a RIF in itself is not illegal providing you have the legitimate reason for doing so, but supplying something like that to someone without them having a legitimate reason for the possession of such a thing, opens both them and you up to potential prosecution, so I'd advise you to at least look at becoming a UKARA trade member if you have not already done so, which would allow you to check any potential customer's UKARA registration number.
I don't wish to stifle your entrpreneurial endeavour, but just be aware of what you are opening yourself up to as far as consequences and the law is concerned. Even the law aside, can you imagine how terrible you would feel if you painted up a gun for someone and then they used it for a crime; not to mention the fact that I suspect some publicity hungry politician would likely have a field day and start trying to stomp airsofting out of existence if that occurred. So please try and keep it as legit as you possibly can by going the UKARA route. You may already have done so, and if that is the case, then you should be at pains to point that out on any potential promotion you indulge in, such as this post. Because you are walking into a legal minefield.
