The casque will often not stay green when they get older. Just be sure that the coloration isn't due to a basking lamp being too close, but it sounds normal to me. Here is a pic of Zaphod's head...
Not really what sure what you are referring to? The grayish looking part on the two little face ridges along with the one going up the casque? (Apologies for terminology) If this is what you are referring to then you don't have anything to worry about. Mine looks the exact same way. I think most every one on here has the same thing going on as well. By the way you got a good looking guy there!