Yes, but Be aware that Colts come in bid hole and small hole versions, so there might be issues mating them up to different uppers/lowers. It's "correctable", but one more hassle. Not to mention, the premium you pay for the Colt.
Shopping used, I've seen complete uppers/lowers for sale $475 total - but that's some serious shopping aroung and you have to be Johnny-on-the-spot and jump on the deal. That's shopping THR adn AR15.com for deals. They're out there. Much more than that, you might be better served getting a kit - under $600 or so. Above that, buy complete used Non-Colt rifle. You can find them all day for $700's.