This is a long read (TL

R) but i am trying to cover as many bases here as possible rather than give you a one liner yes or no.
Lets get one thing straight here UKARA is only a database retailers use to verify if you have a legal defence to buy an Airsoft weapon in order for them to comply with the VCRA 2005, it is not in anyway like a firearms licence, you do not need one to own, transport or use an Airsoft weapon.
Any persons can own an Airsoft weapon regardless of age however there are certain legal restrictions in their sale between retailers or persons as well as how the can be adapted.
Your brother (who is over 18) may purchase any Airsoft gun WITHOUT UKARA so long as the gun is painted in a bright colour which covers more that 51% of the gun.
IF he has a UKARA number (NOT LICENCE) then he may purchase any Airsoft gun in any colour he wishes.
Your brother can then turn and gift you this gun (regardless of colour) to you to keep, own, transport and use at an approved Airsoft site so long as that Airsoft gun meets that sites rules (ie an AEG no more than 350FPS etc).
If it is two toned and you cover it with camo tape for a days play which you then remove after the day has ended, as the camo tape is deemed a temporary modification you have NOT committed any offences.
Offences are only committed IF:
Your brother purchases the gun (regardless of colour) and gives it to you demanding an exchange of payment (money, goods, etc) then HE (not you) commits the offence of a sale of an Airsoft weapon to a) a person under the age of 18 and b ) if it is of a realistic colour he has not complied with the S.36 Violent Crime Reduction Act 2006 by selling an under 18 an Airsoft weapon and ensuring that you have a legal defence to buy the gun from him if it is of a realistic colour.
or
If you or your brother purchases a two toned Airsoft weapon and then spray paints it a realistic colour the you could be committing the offence of manufacturing or converting an Airsoft weapon to a real fire arm or intend it to look real even if it does not fire lethal projectiles.
or
Lastly (and this should be a given) you take said Airsoft weapon regardless of colour into any public place (ie somewhere the public have easy access) without a reasonable excuse (such as going directly to or from an Airsoft event) the you commit an offence under Section 19 Firearms Act 1968, if you cause another member of public fear of violence then you will contravene Section 16a Firearms Act 1968. Whilst it may appear that even transporting the gun is illegal so long as you dont draw attention to yourself and are going to or from an Airsoft event then you will be fine.
I hope my above explanation answers most of you "What if" questions.
I used Google, found and read the following reference material regarding Firearm Offences, mostly from the Crown Prosecution Service (CPS) who are the ones who do the "pressing charges" and mainly from the Firearms Act 1968, if you are arrested for any of these offences chances are your case will be referred to the Crown Prosecution Service who will then decide your fate, whether it is to no Further Action (NFA), caution, charge and bail, charge and remand to court, bail with or without conditions pending further enquiries.
Fun Fact: no one in England or Wales has the right to decide if they want to press charges or not on any offence, its an American (ie US) thing, only the CPS has the right to press charges or not hence why are all cases are Regina (the queen) Vs "insert your surname here" more commonly written as R vs Shizbazki (if i had been naughty).
Reference material with annotations

bare in mind the below offences are for England and Wales only as Scotland has slightly different laws which i am NOT familiar with.
http://www.cps.gov.uk/legal/d_to_g/firearms
This page more or less tells you what a firearm is deemed as and the differences between a real firearm, a RIF and an IF as well as the part about converting guns (even if to look real and nothing else)
The following parts below regard the points to prove for each offence mentioned above as well as the maximum sentence or fine and whether it is a summary only or indictable offence, for most RIF and IFs its is summary only meaning that the maximum prison sentence is only 6 months
http://www.cps.gov.uk/legal/s_to_u/sentencing_manual/section_19_firearms_act/
Not having a reasonable excuse to have an Airsoft gun in public, ie just going out with it for s**ts and giggles. Summary only, max sentence 6 months
http://www.cps.gov.uk/legal/s_to_u/sentencing_manual/section_16a_firearms_act/
Regards the same as above but this is for intent to cause fear so don't show your Airsoft gun to your mate who you meet down the street on the way back from an air soft game or you take it out for said sh**s and giggles, if someone sees it and thinks its a real gun, fears for their life and calls the police, the offence is complete. Indictable only, max sentence 10 years
http://www.cps.gov.uk/legal/s_to_u/sentencing_manual/section_36_violent_crime_reduction_act/
This regards our lovely UKARA, if your brother gets a gun for you and gives it to you but demands payment he commits this offence, it has yet to be tested at court (ie no one has ever been done for it) yet nonetheless it is Summary only with a max sentence of 6 months.