It is illegal to buy black guns without defense, but as for buying a replacement part, I'm not too sure. Many people do it, or have sprayed their guns, but that is usually after they've either got site defense or are registered on the UKARA database. I don't think it is illegal to own a RIF without site membership or UKARA, so you might be alright to buy parts. Don't quote me on that though.
Buying replacement parts can be quite expensive, so it'd be much cheaper to get a can of Krylon ultra flat black (or whatever colour) and spraying a few coats over the two tone - as I said, I suggest doing this once you have a defense.
Another option, which is temporary, is to buy some cheap gun wrap/tape. Comes in all different colours and patterns (standard colours to camo). Just simply apply it to your gun. It can always be removed.