I have seen German RAL8000 metal Hk parts vary from what I would call a very golden brown to a greenish black - even to almost black, for the same part within the same batch (at least what came in a 10 pack) - even more variance alloy to alloy to poly.
FN sort of pioneered the non-match look when it realized that it is impossible to accomplish and the SCAR is purposely made to not match. Part of its charm.
anodizing varies greatly - from Geissle:
"In these pictures below you will also notice that some of the receiver sets have wildly different shades of colored anodizing, and they look wildly different depending on the light source! Why would we even consider selling parts like this? The answer is there is absolutely nothing we can do to make colored anodizing consistent."
"...anodize grows an approximately 0.003” layer of hard aluminum oxide (the same material grinding wheels are made from) on the aluminum part. This layer is grown on the part surface by soaking in a chemical bath while being electrically charged. Next the part is dipped into a special dye bath and the semi porous aluminum oxide layer drinks up the dye to take on its color. Then a hot water solution will seal the dye into the aluminum oxide layer. The resultant color is dependent upon the concentration of chemicals in the various baths, dye concentration, dye mix chemistry, aluminum oxide layer thickness, starting smoothness of the part,
phase of the moon, how many slices of pork roll the technicians had in their breakfast sandwich and my favorite: unknown environmental factors.
Because of this the color on our rails, receivers and other aluminum parts can and will vary from one part to the next, even within the same batch. "
https://geissele.com/support/#anodize-shade-variations-and-finish-methods