Out of curiosity, why is dark grey harder then other colors?
I'll give the quick explanation. I'm also not an expert, so please feel free to correct me if I'm wrong.
Basically certain alloys of aluminum take anodization better than other alloys. 6061, the kind (most likely, LZ hasn't said what alloy he used) used in this GB is a "NON-AQ" alloy. Non-AQ is the AAC (Aluminum Anodizers Council, learn more at
http://www.anodizing.org/?) designation for alloys that aren't good at taking anodization.
More info about which alloys are good for what:
The thing that makes dark grey hard specifically, is that to get it right, you basically have to anodize the object black, but not too black. It stresses the aluminum out, and can cause discoloration.
Now, you might be wondering, "Puddsy, why doesn't every GB run in 6063?" Well, funny you should ask that.
MIRA SE (I was confused with the OMIRA, the Chinese one, but the principle still applies) is being run in 6063, but if you look at the lunar grey color (Dark grey with a green hue), you'll see that it's not perfectly dark grey. Every alloy adds a bit of a hue from the non-aluminum metals that are mixed in. 6061 adds a dark grey hue. 6063 adds a green hue. Also, because 6063 is an AQ-type, it's more expensive than 6061, so the overhead costs are smaller.
Hopefully that makes some semblance of sense.