I think the process for grey or silver is different from the others.
Im quoting Puddsy from the CLS thread:
"Dark grey is a VERY hard color to do right, especially with this alloy of aluminum (6061), so I suspect that's why it's not quite perfectly grey. This is actually the reason the MIRA uses 6063, so that they can get the lunar grey to look right.
Every alloy of aluminum gives a slightly different color overtone, and every anodizing pool gives a slightly different color. That's why sometimes you'll see Chinese GBs talking about "bad pools." The benefit to 6061, however, is that it gives dark grey overtones to whatever color you anodize it."
Why small variations in color?
"it's because it's a "chaotic electrobonding process," and there's no "canned" grey color like with cerakote. You're basically forcing the aluminum to be dark grey, and inconsistencies just show up more often in that color. "