Need tips on how to keep buttercream covered dummy cakes from turning yellow. I know not to use butter, just straight crisco. They are still turning yellow after a few months. HELP!

Use joint compound instead, I'm not kidding. It will give you the look of buttercream but will be permanent. They make artificial "icing" for decorating that's basically the same thing as joint compound or spackle.

How about covering your dummy with a thick layer of saran wrap before applying the spackle, then you may be able to remove it after a few months to try another display?