Hi Andrew,
Having researched my 1/8 Panther A to the height of boredom prior to painting, the usual color I have discovered for the jack is the color of the vehicle.
I had the chance to view an original unrepainted jack some time ago and I remember it was in dark yellow, with very heavy wear to the paint and showing a red (I assume primer) underneath.
There is a single photograph that I have also seen showing the jack (Panther Ausf. A) also camouflaged within the pattern of the vehicle. I suspect this is unusual with in this case, the crew or the field workshop being too busy/lazy to remove it prior to painting.
Hope this helps.
Best wishes.
Trackpins (Peter)