First, if I was looking to do a historically accurate dio, I'd check what unit the Marder was marked in. Then check the unit history as much as possible to see if there would have been panzergrenadiers assigned to that unit, looking at the unit history, determine where thy were depolyed before 1943 when dunkelgelb replaced dunkelgrau. Finally, if you're depicting action, and I think your figures are in action, determine if an anti tank vehicle would have been tactically deployed with grenadiers.
Of course, you're also free to choose any setting before 1943, Eastern front, France, Poland, the low countries and be creative. NO one can absolutely prove a negative, i.e., you can't prove there was never an instance where a grenadier didn't jump off a Marder III, only that there you have found no evidence that it did happen.