In certain sentences, I use that is to rephrase something, be more specific or elaborate.
This coffee is so black, it must have been prepared by someone who works a lot, that is: an insomniac.
But in certain circumstances, this colon is unpractical because there's another colon around. Or it makes the sentence insist too much on this particular point.
And I think that I already saw that is without a colon.
Is this punctuation sign required?