Hi again Dave,
of course You are correct to assume that Olivgrun and Rotbraun can be overpainted to the factory applied Dunkelgelb, as was Ordered.
I suggest You to search for some photographic reference, as precise as You want, from a single Sd.K.fz.250/9 ausf. A to a "generic" field application of German WWII camuflage in 1943-44.
Of course Sd.Kfz.250 Ausf. A exited from production a lot of time before factory camouflage patterns were introduced, so Your options are two, plain dunkelgelb or dunkelgelb with an overpaint of other colors (expected Olivgrun and Rotbraun).