I am in the process of buying a Colt blue label receiver.  Because Colt M4 uppers are pretty expensive, I was thinking about a new Preban Bushy M4 upper, but I wondered if the black finishes would match...or maybe I should just stick w/ a new preban Colt M4 upper.  BTW, how would I know which ones were RAS marked...are there any serial numbers.  Other than Colt and Bushmaster being different companies, how can I tell a Colt M4 upper from a Bushy M4 upper by looking at them?  Does the Colt piece have the logo?  Thanks

Color wise, the two parts will not match.  Blue label Colts are med/dark grey in color and Bushmasters are black.  Functionally they will work fine (although you may need an adapter for the front pivot pin depending on the Colt lower you have).  Also, even a new Colt upper will be black in color.  You'll need to find an older grey Colt upper to match.

Secondly, you will be able to tell a Bushmaster M4 upper from a Colt because the Bushmaster will have a BMP 1/9 on the barrel and Colt will have CMP 1/7.  Also I believe a true M4 Colt will have a "4" over the gas tube hole.