MM '43 Panzerschwarzgrau is the most accurate representation of German Panzer Gray IMHO. I've used the enamel variety and it has captured the bluish tint to the gray very well judging by the leading lights of the armor world at any rate...the problem with RAL colors is that it's very hard to exactly duplicate them so each manufacturer has a slightly different take on it.
One thing to note about German armor schemes, if you are interested in accuracy in this department, the Panzer Gray was phased out in early '43 and replaced with a Dark Yellow as the basecoat. According to Achtung Panzer! http://www.achtungpanzer.com/hetzer.htm the Hetzer didn't enter production until April '44 and 2,584 were produced from then until May '45.
This link shows some schemes for the Hetzer down towards the bottom of the page:
http://bsdi.usppp.com/wwiiscalemodels/schemes/panzertarnung.html