I grew up watching that excellent Great Planes documentary on the P-51, as my dad had recorded it on VHS sometime in the very early 90's - it was the American version however, that was broadcast on the Discovery Channel, and called "WINGS" instead, and although the footage and the script was the exact same, they had an American narrator instead. It was fun when I first saw that original version of the documentary posted to Youtube, to be able to relive it all over again - probably the best film documentary on the P-51 ever produced (not to be confused with the awful (load of crap) "Great Planes" series that was made for the Military Channel - in which they rehashed the original script, and mixed it with footage that doesn't match-up to what is being told).
One of my favorite parts of that documentary is the color footage that was shot at the North American Inglewood plant, in the summer of 1945. All of the Mustangs shown are P-51D-30-NA's (built too late to see combat in WWII), some of which even are fitted with the non-cuffed square-tip Hamilton Standard blades, which were introduced into very late P-51D production, specifically during the D-30 production block. In some of the scenes you can see the wings with their dark grey filler applied (and sanded), before the wings were painted silver (it seems that test flights occurred before the aircraft received their final paint (silver wings, stars and bars, serial number)). You can also see in these shots the black temporary stars-and-bars applied to the wings, which were crudely done while the aircraft was still being manufactured, before being completed and moving into actual paint (serving as a reminder to the paint guys, the proper sides of the wings to place the stars and bars). The numbers on the sides of the fuselages while the aircraft were being manufactured were the aircraft's individual factory number, and not related to the USAAF serial number. With the right resources at hand, however, you can actually find out what the aircraft's USAAF serial was, just by the factory number and if you know what the production block is.
Also, a detail that might otherwise go unnoticed, at the 0:42 mark, you can see a then recently completed P-51H awaiting takeoff in amongst the factory completed P-51D's (late enough, that it has the extended vertical fin cap, designed in conjunction with NACA, I believe first introduced on the 13th production P-51H (where as the earlier ones first came with shorter verticals)).