I'm not even sure whether vapour colouring is banned, or whether it is not around because no one has as yet found a viable way of colouring it.
Liquid colouring was mooted at one point as being against the TPD regs, and I seem to remember reading somewhere that it was, yet here I am with a bottle of Element Liquids juice in front of me that is a light orange colour.
Whilst I have on the odd occasion thought if someone could make a coloured vapour juice "that would be a neat trick" in reality if you were to use one outside your own house it would give vaping more of a bad press. There are several pubs near me that will let me vape in / at the bar, but I bet the landlord would be less amenable were I producing red, or purple vapour.