If the base color is Dunkelgelb, the engine compartment is Dunkelgelb, unless this was a field workshop repaint of a Dunkelgrau vehicle, in which case the engine compartment would most likely not be repainted and would be left in it's original color.
Since Kubelwagens served throughout the war, in all theaters of operation, the full range of German color combinations are possible.
I have color photos of kubel engine compartments.