With modern engines you will find that most of the greasy and ugly metallic areas are covered by plastic trim. This looks so much better when it is colour coded to match the rest of the car. If you are painting your existing engine cover you should use heat resistant paint otherwise it will be prone to discolouration. If the plastic surface is smooth then it will need to be keyed to allow good adhesion. You should also ensure that any grease or oil from the engine is removed as this will cause the paint to crack and fade.