In addition to brown, already mentioned, copper may oxidize red or green (or a combination of two or all three. Yeah, washes will work as will dry brushing or even careful very thin airbrush coatings. Actually, a combination of the red and brown can give close to a rust color, but it must be just a subtle patina.
The green is usually a water caused patina, so for the engine the browns and reds should predominate- very little or no green.