Never keep the dual color wild property cards on the top of a two+ card stack (hide the wild). The objective is to keep your opponents knowledge of your cards, Bank, and hand to a minimum. This includes the cards you already have on the table.
Strategy Examples:If you have two Railroad cards on the table stacked together and one is a Railroad/Dark Green Wild Card always place that card under the other Railroad card so your opponents can’t see the Dark Green Wild and won’t know if it’s worth taking. (Remember opponents can’t touch your cards so they can only ask for what they see)