I'm working on Dragon's Panzerbeobachtungswagen III. I understand that it is an artillery observation tank. Would the crew wear the black panzer uniform, or a fieldgray version?

I believe it would be fieldgrey if the vehicle was part of a Stug unit, but what if they were assigned to an artillery regiment for a panzer division?

I can't find any definitive photos.

I'd appreciate any guidance...

I've seen pictures SS Divisions in Kirsk with dot pea cammo.
Others looked gray to my eye. The commanders had gray and the crew black? This one is 2nd SSPanzer Das Reich with dot pea cammo. HTH---John
2nd SS Panzer Das Reich by jvandeu53, on Flickr

The Das Reich commander in the photo is wearing the reversible overalls in the "oak leaf" pattern green one side, brown the other. These were standard issue to SS panzer crew at this time and were also often worn by recce crews and would be eminently suitable for an SS Panzerbeobachtungswagen crew. DML does a set of figures wearing these overalls, entitled "LAH at Kursk".
Heer crews, as opposed to SS would wear the standard Field Grey Assault Gun uniform, as did the SP gun crews.Red Waffenfarbe,as the crew are artillerymen,not panzer.
The Panzerbeobachtungswagen III was supplied to Stug, Hummel and Wespe battalions.This one appears to belong to the 7th Battery of SP artillery according to the tactical sign, which implies that the SP battalion in DR was the IIIrd, although David Porters Illustrated Battle Guide - Das Reich at Kursk, shows it as the Ist.
Later there were Panzer IV & Panther versions, also modelled by DML.