From what I have read it seems that anyone over the age of 18 can buy a two-tone and gift it to someone aged under 18. Then, providing the under 18 is a regular skirmisher (at a recognised site?) they can the either repain it to make a RIF or buy replacement parts for the painted ones.
Similarly, an over 18 without a valid defence could but a RIF that has been two-toned and also buy the bits to convert it to a RIF - but that would then be illegal. Does not prevent them easily getting a RIF without a defence though.
With that in mind, the system that Xsite run which allows the parent of a regular skirmisher aged under 18 to buy a RIF seems far better for the sport as it does require the person who will use the RIF to be a skirmisher. It also reduces the cost to the customer as they do not pay for a painted RIF and then have to buy the bits to change it back to the original state.
The only downside for the customer is that they have to buy the guns from Xsite in a face-to-face transaction rather than being able to shop online but they do benefit from an excellent back-up service.