I was looking for a Colt 20" AR last March for my first project of this year but can't find any so I built it.
I started out with a Colt M16A2 brand new surplus GI upper.
Then I found a Colt post ban Match Target HBAR stripped lower receiver that my local dealer had.
I got all Colt LPK, CH, M16 BCG and A2 stock and put it together in about 30 minutes.
My lower didn't have the push pivot pin but there are some Colt post ban lower that have it.
I'll have my push pin holes added later on by ADCO when I have time.
The color mismatch doesn't bother me but you could either match the upper to black or the lower to Colt grey as you wish.
Here's a pic of my A2 in the Industry Colt pic thread, about in the middle of the page.
Good luck.Colt pic thread link