"What colour does not exist in the afterlife?" answer to an ARG riddle

So there's an ARG that started on the Unfiction forums where you have to email a secret to a particular email address. One member has done this and received:

"CLEVER?

6* 83:.; ]45; 59 6

The only real headway so far is that it may be some sort of keyboard substitution cipher, but what I'm really looking for help with is the riddle I received when I emailed it, which is completely different:

"IDENTITY IS IN THE EYE OF THE BEHOLDER.

ANSWER THIS: WHAT COLOUR DOES NOT EXIST IN THE AFTERLIFE?"

A couple of the suggestions I've received so far have been "green" (the colour of life), "black" (the colour of nothing, not on the RGB spectrum) and "time" (assuming the answer is more metaphorical) but honestly I don't have a clue. Anyone have any ideas?

• Is this an ongoing contest? Jun 16, 2017 at 12:59
• @Forklift As far as I know, yes. The email address is still active and I assume that once I manage to figure out the answer to this I'll be sent another clue. Jun 16, 2017 at 13:02
• Our site rules actually prohibit soliciting help for ongoing contests, I'm afraid. puzzling.meta.stackexchange.com/a/1675/36638 Jun 16, 2017 at 13:05
• It's not a contest though, there's no prize. Jun 16, 2017 at 13:11
• Hmm. Not sure about that. I will not vote to close then :) Jun 16, 2017 at 13:12

OK so I think this is a completely different puzzle and then poster thinks.
IDENTITY IS IN THE EYE OF THE BEHOLDER is the clue

IDENTITY (subset)) THE EYE OF THE BEHOLDER and what part of COLOUR is not a subset of THE AFTERLIFE

here is a sample solution I can think of. We use the set as a range of letters so THE EYE OF THE BEHOLDER has letters that span the alphabet from E to Y every letter in IDENTITY fits in that range

THE AFTERLIFE has letters A to R What part COLOUR does not fit, note how it's oddly spelled British?

The U

I'm almost certain that the first sentence is related to your "secret", as mine is "THE PAST IS UNCHANGING. FORWARDS IS THE ONLY WAY." So as far as I can tell, that's not relevant to the question, but I've been wrong about these things before.