You're absolutely fine to change the colour. It's still an IF, so no offence is being committed.
On the second point, I have to be pedantic and note that it's never "legal" to modify an IF into a RIF. It's always an offence, just as it's always an offence to sell, manufacture or import one. Your need for a defence only kicks in if you are prosecuted for it. To the best of our knowledge, nobody ever has been, and so we don't actually know what would constitute a defence.
Having a UKARA number is generally viewed as the safest option. Border Force do seem to accept that is a defence, and they treat it as though it were a licence to import (although it is not, as they acknowledge).
Personally, I ensured that I had another airsoft skirmish booked (and deposit paid) before removing the two tone. I believe this actually more closely satisfies the requirements of the defence, which is to show that the purpose is to perform the nebulous "permitted activity" (airsoft skirmishing) in the future tense. UKARA only speaks to past behaviour.
The tl;dr version is that it's entirely up to you to decide if and when you modify your IF into a RIF. There's no case law, no guarantee of safety, but there's been no prosecutions that we know of either.
The tl;dr version of the tl;dr version is: sure, get UKARA, strip or spray.
