Out of the jar, XF-18 is closer to the blue-grey in the US Navy's early WWII camo scheme, and close to AS-12, too. I use AS-12 for that color, too.

XF-17 is a little darker than either AS-12 or XF-18.  But looks like you've got a handle on it.

It depends on whether you worry more about exact colors, FS numbers, and the like.  Personally, I go by the Mk 1 eyeball and whether it looks right to me or not.  I look at the colors as starting points, anyway, which will get weathered and that changes the look a little, too.  Olive drab, same thing-so many different shades, so many variables-I just look for as many photo references as I can, and then go from there.