Hi Ross,G'day,
There are a few little tricks to enhance the beetle green sheen as Ron is absolutely correct. The oil glands secrete and promote the sheen.
I use corn oil and another oil that is used to enhance the coats of show horses mixed sparingly in their feed. I'll feed this to my selected show birds up to four weeks prior to a show and that enhances the sheen. You have to be very careful not to give them too much as it can sometimes send them into a forced moult. Good Australorps should have a natural beetle green sheen.
Regards,
Ross
think I posted this earlier, but interested in your thoughts. Comes from an old book called "The International Poultry Book". "To secure more colour in the plumage of Black Orpingtons and other breeds requiring green sheen, the feeding of sulphur before the last feathering is recommended. Start with a saltspoonful a day and increase the dose to up to a teaspoonful, to be given in mash. Done in late summer to mid autumn(fall)".
Cheers Geoff