There are quite possibly a million posts on every forum covering exactly this topic. Every new person asks exactly the same questions, but I’ll dive in and give an answer.
1) It’s not a UKARA licence. It doesn’t entitle you to do anything, you don’t need to pass a test or apply for anything. You go to an airsoft game site, become a member, you will get a membership number for that site and once you have played 3 games in no less than 3 months The site will endorse your membership. You can then buy a R.I.F. from an airsoft shop who will ask for your site membership/UKARA number, confirm it with the game site and then put your number on the UKARA database meaning any other retailer will then be able to check up on the UKARA database and confirm you are an active skirmisher. You can then buy as many guns as you want in that year period before your membership expires or you renew your membership with that or any other site, having hit the required 3 games.
2) It’s down to individual sites, but I’m sure you will be able to become a member of your local airsoft site and get a membership number below the age of 18.
3) As well as being on the UKARA database, you need to be over 18 to buy a R.I.F or a I.F. You will need to prove you are over 18.
4) The law states you cannot convert an imitation firearm (I.F.) into a realistic imitation firearm (R.I.F.) that includes changing the colour of a two tone gun. Are you going to get locked up for stripping back a two tone I.F.? No, unless you ack like a d*** and go running around the streets with it, posting pictures of it on public social media or in some other way cause the general public harassment, alarm and distress. If you have an I.F., strip the bright colours off it and turn up at your airsoft site with it, I doubt anyone will have an issue. That said, converting an I.F. To a R.I.F. is still illegal.
Hope this helps in some way.