Depends on lighting: Is the item at top or bottom of the engine bay?

Are you painting aluminum casting, aluminum sheet, aluminum extrusion, pot metal die casting, cast iron, forged steel, cold rolled steel sheet, hot rolled steel shape? As cast, as machined, as rolled, etc.?

There is a thorough testing of many colors Here

One option for bare steel is to leave the item actually bare metal by applying phosphoric acid, wiping with mineral spirits, and coating with 50-50 Cosmoline-mineral spirits. This may last as long as a year or much more. In case of bare steel, later minor surface rusting is easily fixed by simply repeating the process starting with rubbing the item with Scotch-Brite pad or similar wetted with phosphoric acid.

Before and after photos show treatment of bare aluminum that was deteriorating and showing powdery "white rust" particles. The treatment was dry brushing off the white rust, wet brushing with soapy water, and after drying, application of Henkel Alodine 1201 yellow. The photos were taken after the white rust particles were removed. Alodine is mainly for treating and preserving bare aluminum aircraft parts. It comes in yellow or clear. Can also be used on zinc plating.

I believe a few Mopar bare aluminum parts of the late 1960s-1970s were treated with yellow Alodine or similar. Examples were starter and alternator parts.