The use of Grey-Green, RAL 7009, was discontinued after late 1942 as an interior color, however, stockpiled parts could still be found in this color at later dates, this according to; http://byrden.com/panzers/Colours/. I'm certain however, that I have seen this color on parts of much later vehicles, (the Littlefield Panther A restoration in particular comes to mind, as well as some of the other museum pieces I saw in Germany). It is a standard color for all sorts of equipment, (esp. railroad and electrical equipment), in Germany to the present day. I'm a little wary of Mr. Byrden's date on this subject.
For completely open vehicles, like a SdKfz 251/1, etc., everything visible inside would be painted the same as the exterior, except the gauges, seat covers, steering wheel, and various knobs. I have color photos of both early and late model 251s that have been fully restored, with completely Panzer Grey or dark yellow interiors respectively. Apparently there may have been some very early open topped vehicles that had Elfenbein driver's compartments, but this was quickly dropped, as it compromised the vehicle's camouflage from the air. I have not personally seen photos of this however.
SdKfz 234 series gets tricky. There is a 4 month period between Sept. 1944 through Dec. 1944 that the use of Elfenbein was discontinued and everything (but the knobs, steering wheel, etc.) was left in Red Oxide Primer. This period of red oxide interiors lands squarely in the middle of 234 series production, so it is possible to find them with completely red interiors. The 234/4 I saw at Munster had a completely red interior, at least what I could see, except for those parts, like the gun shield and upper superstructure interior, which was dark yellow.
Once the lunacy of red interiors ended in Dec. of 44, Elfenbein was re-instated for closed-top AFV interiors. This would include about 1 month worth of production of the 234/1 in Jan 1945. These would have dark yellow turret interiors, but Elfenbein interior walls and red oxide floors.....Theoretically.