Anti-tank guns were painted the same colours as vehicles. Up to the switchover to dark yellow in early 1943, all of these guns would have been painted dark grey (the Pak 36 would have had a red-brown camo pattern in the intial year or so of the war). They were often whitewashed in winter - usually very roughly. Otherwise, they were camouflaged with foliage or even mud (common in the early states of the desert war). Guns in service with the DAK were eventually painted brown yellow, although not always uniformly (a rough spray over the dark grey wasn't unusual).
With the change to dark yellow, all equipment was repainted, but it took time. Crews often applied camo patterns to Pak guns - after all, camouflage was probably even more vital for an anti-tank gun than it was for a vehicle. As a rule, they tended to be quite basic schemes (often just a quick sprayed pattern of either green or red-brown), but you do see some more elaborate 3-colour schemes. Guns serving in Panzer divisions seem to have been more likely to be properly camouflaged than guns serving in infantry divisions. If the gun was towed by a half-track then it tended to be camouflaged the same way.
I found this (genuine) colour photo of some German heavy artillery captured in Tunisia. It gives an idea of the kind of slapdash application of camo that was pretty common on towed guns (and the actual colours).
Here is a photo of a Pak 38 with a more elegantly applied camo pattern.
And here is a Pak 40 with another fairly typical camo pattern.
