They don't have to be any specific colour at all. The VCRA states that a R.I.F is anything that any reasonable person could mistake for a real firearm. Therefore an I.F (perfectly legal) is anything that is so designed as to be easily identified by any reasonable person as being a toy.
So any bright colours are fine, but the majority of the I.F MUST be brightly coloured.
The only people who can sell RIF are licenced retailers (basically Gun shops or airsoft shops) and the only people who can buy them are people who are registered as members of a insured airsoft organisation or possess a firearms licence.
In short if you purchase a RIF from a market stall or any other shop, both you and the retailer are breaking the law.
Of course, then there is the fact that if you carry a RIF in public place you can be arrested and charged under the firarms act. Again, you only have to do something that causes distress to the public to be in violation of the firearms act, it doesn't matter if said firearm is real or not.
Hence why the whole VCRA thing is a pointless entity and just something to appease the ignorant masses as usual.