I agree that those lava tubes would make fantastic habitats. (I try to avoid "colony" for its political implications. Sentimental for "The Moon is a Harsh Mistress") I'd guess that as long as the roof is thick, and the rock isn't badly cracked, you've got a ready-made pressure vessel. A pressure vessel the size of several Moffet Field dirigible hangars laid end-to-end.

And they have skylights! People pay good money here on Earth to cut skylights in the roof.

But seriously, there's one thing about the idea that I find particularly compelling: Elon Musk is hallucinating when talks about sending millions of people to Mars at $200,000 each (household?). Not in this millennium.

But the Moon is one hell of a lot closer, and with ready-made habitat structures available, a SpaceX heavy-lifter might well be able to send settlers to the Moon at a reasonable cost. And to people who live in areas with hyper-inflated real-estate, a million bucks for a condo on the Moon sounds like a dream home in one-sixth G.

A home on the Moon might sound pretty dreary if you picture a can buried under the regolith, the way NASA PAO illustrators picture it. But a space the size of one of those lava tubes (with a skylight!) could be made really habitable. A light and airy space for two weeks, then a night sky for two weeks with the illumination on the floor tracking the normal diurnal cycle. That seems to me well within the range of comfortable adaptation for humans. No rougher than living in Scandinavia. With 1/6 the gravity, but colder outside.

Sign me up, man!