Although you cannot get a UKARA registration if you are under 18, you can own a Realistic Imitation Firearm if you are under 18 and it does not have to be two toned either, although I'm sure you realise that it is advisable to have a suitable case for any airsoft gun (two toned or otherwise), when traveling to and from skirmishes, and not to have the magazine or gas or battery in the thing when carrying it about either (so it cannot be fired at the time), particularly if you are going to use public transport to get to a skirmish site. These might not be real guns we are using, but they are realistic enough to get you into trouble, so there's a bit of responsibility which comes with owning one.
It's worth noting here that the UKARA Scheme is, as you know, more about airsoft shops covering their own asses legally than anything else, so a legitimate defence for owning an airsoft gun would in fact be to have a membership card for an airsoft site, as this would clearly indicate that you are someone who goes airsofting at a legitimate venue regularly. You might (and I stress might) find that an airsoft shop would accept that as suitable enough proof to sell your parent/guardian an airsoft gun to give you as a gift if they accompanied you to a shop (particularly if it is a site shop - such as the one Trojan have at their Mill site - where they know you are not a dickhead and are going to use it for airsofting), although here is no guarantee that this would be the case of course, but it would be perfectly legal for them to sell your parents an airsoft gun under those circumstances so long as the shop was satisfied as to what the gun was going to be used for. To explain that a bit more...
Despite the fact that you can own a RIF, you cannot buy one yourself if you are under 18, so any RIF you do own would have to be gifted to you by someone who did have a UKARA or other legitimate defence for having bought it (and if your parent or guardian does not do airsofting, they are unlikely to have a UKARA, which makes it a bit tricky). No money can change hands for such a transaction either, i.e. you cannot give someone who has a UKARA the money for a RIF and have them buy it for you, it has to be a gift where no money changes hands. Although the law is not actually very clear on this matter, one assumes this is to enable parents who are into airsoft and who do have a UKARA or some other credible reason, to be able to buy RIFs for their kids (yeah I know, the law is fairly stupid and it could be got around relatively easily, but that is nevertheless the law).
So in short, no, if you have a RIF, it does not have to be two toned, even if you are under 18, although to be honest, the blue/black two toned guns actually don't look too bad, so it's really not a big deal, and there is nothing stopping a parent buying you a two toned airsoft gun. But I guess the really big deal is in persuading a parent or guardian to let you have one in the first place, since not all parents are exactly in love with the idea of their kids having fairly realistic weapons. It's a bit of a shame that more of them don't realise that airsofting is not only very sociable, as well as healthy from an exercise point of view, so actually quite good for kids, who these days are quite likely to be indoors all the time on a Playstation or X-Box (so neither being very sociable, nor getting any exercise), but it is also inclined to teach younger people a bit of responsibility.
The way around all that difficulty if you cannot get a RIF or a two toned airsoft gun, is to hire the gun, which is usually only about a fiver or so if you are a member of the airsoft site you go to regularly, although you should be careful to point out to your parent or guardian that this is what you are doing, as it is inclined to make them realise that letting you have your own airsoft gun would in fact save money in the long run. But before you get to all that, just hire stuff and see if you actually like airsofting. It is good fun, but you'll only know if you enjoy it by actually doing it a few times.