The drivers of the SdKfz. 250/251 wore the standard field grey field uniform of the army (field blouse etc.) with one peculiarity: Initially, the black protective beret as used by tank crews was to be worn for driving duty. This unpopular item was hardly worn anymore by tankers after ca. 1940 or so, and it is safe to assume that halftrack drivers didn't retain it any longer.
Military vehicles and their allocation and use are not exactly my strong point, but as I understand it, these half-track APCs were normally used for carrying Panzergrenadiere into battle. This means that their drivers were usually eligible for the Panzerkampfabzeichen in Bronze [Panzer Assault Badge in Bronze], which was awarded to personnel of the Panzergrenadier regiments, motorcycle battalions and armored reconnaissance units of Panzer divisons. The branch color [Waffenfarbe] for Panzergrenadiere was officially designated as "wiesengrün" ["meadow green"].
(If such vehicles were allocated to other types of units, their drivers would have been eligible for other types of assualt badge and have worn different branch color, but, as I said... not my strong point.)
Halftrack drivers had to undergo an additional qualification course and test indeed. There were at least two different categories, namely for half-track vehicles up to 10 metric tons and half-track & full-track vehicles over 10 tons. ["Halbkette bis 10 Tonnen" and "Halbkette und Vollkette über 10 Tonnen", respectively, are the categories that I am aware of.]
They could be awarded the motor vehicle drivers' badge [Kraftfahrbewährungsabzeichen], although this was not a qualification/trade badge, but a personal award for which drivers became eligible by meeting a fixed number of days in action under difficult conditions (the number being different for various categories of drivers) while deployed to certain operational areas during designated time periods. (An APC driver qualified for the bronze grade after 120 days and for the silver and gold grades after an additional 120 days each following the previous award.)
Thanks you so much Andreas, Bob, and everyone else who have commented.
I have taken on a role of a halftrack driver and I want to make sure my uniform is going to be 100% correct. I will be in charge of an OT-810 (postwar Czech Hanomag copy) which has been converted to resemble an Sd.Kfz. 251.
This is the particular beast in question:
No parking? Tow away zone?
" When you're chewing on life's gristle, don't grumble, give a whistle "