I think people maybe have an unrealistic level of expectation on color matching of these newly made "retro" ar parts, specifically the aluminum ones.
Anodizing is a chemical process that depends on the chemicals, electricity, an aluminum alloy, and the time it is in the bath to determine the color of the anodizing.
Even a slight variation in any of those factors can change the color. And as the bath is used multiple times, this changes the bath itself.
In point of fact, when anodizing in a color (gray, FDE, green) anodiziers do not provide a single color, but rather a range that the color will be within. How do I know this? We work with one of the largest OEM anodizers in the nation who currently does the work for many of the big companies including Colt, and when we have done anything but black we provide them with several sample pieces of the part we are making, and they send us back parts to show the light to dark end of anodizing color range.
The true retro parts I have seen have plenty of variation in the shade of gray, and true guns even have variation between the upper and lower.
The best I can say is if you want matching, get repro retro uppers and lowers both from Brownells. Otherwise understand that even the true guns had variation in their color, even between their upper and lower, and it is a bit of chasing a unicorn not only to expect the colors to match exactly, but also to think that is how the true guns were.
Anyhow, that is my two cents on this topic.
Sven
Manticore Arms