Early Nazi German vehicles wore the Reichswehr scheme of panzer grey, dark green and matt brown. This was latter replaced by plain panzer grey - even vehicles used by the Afrika Korps were delivered in panzer grey, although build up of dust or applications of mud by the crew can make them look sand brown.It was only in March 1941 that a sand yellow paint was introduced.
A useful website for markings is:
http://www.deutschesoldaten.com/vehicles/markings.htmThe sdkfz 222 would have sported WH number plates, balkenkreuz and the armoured car tactical sign (see weapon branches in the link). Tactical numbers seem to have been optional. As for unit markings - totally up to you as its a pulp background.
Given that Spain had African colonies, another option could be Condor Legion markings, but I'll leave it up the Spanish Civil War experts to give advice on paint/markings