The main problem is that most photographs of the period are in black and white, there are a few in colour showing these vehicles but I cannot remember where I have seen them. RAL 6009 is fine to use and indeed, so would any darker shade right up to black itself. One thing to note is this, normal Police cars did use POL on the number plate (so did the Feuerschutzpolizei) however Gestapo cars where not meant to be noticed and would have used standard civilian plates. This can be seen quite nicely on vehicles such as the Citroen Traction Avant used in France during the occupation.
Also if any of you guys wish to use different number plates on your vehicles the following list could be useful:-
WM: Wehrmacht-Marine (Kriegsmarine)
WH: Wehrmacht-Heer
WL: Wehrmacht-Luftwaffe
RP: Reichspost
DR: Deutsche Reichsbahn
Pol: Polizei and Feuerschutzpolizei.
SS: SS-Verfügungstruppen and SS-Wachverbände
OT: Organisation Todt
RK: Reichskommissar
MB: Army Commander Belgium and Northern France
MD: Army Commander Denmark
MF: Army Commander in France
MG: Army Commander in the General Government
MH: Army Commander in Holland
MN: Army Commander in Norway
MO: Army Commander the East
MR: Army Mission Romania
MS: Army Commander South
MU: Army Commander Ukraine
ZB: Civil Administration Belgium and Northern France
ZF: Civil Administration in France
ZO: Civil Vehicles in area of operations East