I have a Panzer IV ausf. D in my stash. While examining photos and details of the variant, I happened across the fact that the ausf. D original order was cancelled. My question is, was the ausf. D ever fielded and, if so, in what theaters did it see action?

Looks like it was, top image.

http://www-d0.fnal.gov/~turcot/Armour/pz3.htm

According to von Senger and Ettelin's book D types were "field tested" in Poland and declared fit for duty in late Sept. About 45 were built.

This line from another article would explain the cancellation you mentioned: "In response to the difficulty of penetrating British Matilda Infantry tanks during the Battle of France, the Germans had tested a 50 mm (1.97 in) gun—based on the 5 cm Pak 38 anti-tank gun—on a Panzer IV Ausf. D. However, with the rapid German victory in France, the original order of 80 tanks was canceled before they entered production." Instead, they just brought out the "E" model.

According to Doyle & Jentz in Panzertracts N°4, from October 1939 to October 1940 only 232 of the 248 PzKpfw IV Ausf. D contracted to Krupp Grusonwerk as the 4 and 5 B.W. were completed and accepted. The last 68 were completed with 50 . mm thick frontal hull armor. One or more of the last Ausf. D were assembled with 6 B.W. (Ausf. E) superstructures and turrets.

RZV

The 45 I mentioned were the ones that were built prior to Poland. After going into further searches, that became apparent. Apologies for the misunderstanding.

So, were the 80 tanks ordered cancelled as part of the larger order? I'm really looking to give it something other than a Panzer grey finish. It does look like they were only used in Poland and France, both of which would suggest the overall grey finish.

Lots of conflicting information here . Here's what I got ...
Encyclopedia Of German Tanks ( Chamberlain , Doyle , Jentz )
Krupp - Grunson given order in 1938 to build 200 4th series BW and 48 in the 5th series .Total gun tanks completed - 229.
The rest : 16 Bridge -laying tanks ,2 SPGs ,and a Munitionsshlepper for Karl. In 1941 one Ausf D rebuilt with 5cm KwK39 L/60. In 1943 several up gunned with 7.5cm KwK L/48 for training ( HJ in France 44 ? ).
Ausf D served in France , the Balkans ,Africa and Russia , last few phased out by attrition in early 1944.

According to German Tanks of World War II by von Senger and Etterlin, only limited progress was made to develop the Pz. IVs due to the industry's low production capability. I have seen photos of the ausf. D conscripted to the Ghost Division and several with typical panzer crews. Even near only 250, the ausf. D appears to have been a rare tank on the battlefield. The kit I have is of the shorter barreled variant. I find photos of this even less common.

While they weren't numerous you can pretty well figure the campaigns in France and Poland didn't reduce the number by that many. If they made it into Africa, then you are open for a desert scheme. Any surviving into '42 and beyond could have feasibly been repainted dunkelgelb as part of factory overhauls. Some of the longer lasting ones may have even sported a camo pattern. I'd say you're pretty much open to your imagination (except the late war scemes).

As for the barrel length, pretty much all the "D"s had the short barreled 75.