Interior components weren't painted in Panzer gray as a rule...there was no need for it and a separate specification existed for those components. Since Ausf D production ran from 1939-1941 and that spans when the interior primer used was transitioned from a field-green shade primer to red oxide, you could go with either green or red for the fans and be correct depending on when your D was built .
The fans in question would never be exposed to outside view unless the rear deck hatches were opened up, so the interior painting guidelines would apply. HTH!