That is the correct color. It is flat gray. Prior to decaling, spray several light coats of Future over the model. Let the future dry and then put on the decals. Decals work best on a gloss surface. After the decals are dry shoot future on the model again to seal the decals. After all is dry you can spray a dull coat on it to bring it back to the dull state. The actual aircraft had a very dull finish which would get very dirty from everyday use.
Berny
Phormer Phantom Phixer 
On the bench
TF-102A Delta Dagger, 32nd FIS, 54-1370, 1/48 scale. Monogram Pro Modeler with C&H conversion.
Revell F-4E Phantom II 33rd TFW, 58th TFS, 69-260, 1/32 scale.
Tamiya F-4D Phantom II, 13th TFS, 66-8711, 1/32 scale. F-4 Phantom Group Build.