Got this off the Fox Collectors website:

From my observations there are three distinct types of case colors on Fox guns. The early type bone charcoal colors are illustrated above. The second type is seen on guns from the 1916-1918 period and have more light straw color than vivid blue. Doug Turnbull told me the more straw color the harder the surface. These guns exhibit very subdued blue. My theory is these guns are charcoal blued using a chromox frame and they hardened differently than the earlier non-chromox frames. The last group is the cyanide colored chromox frames that have a dark blue appearance.

PS. I think they have been redone.

foxes rule