To produce a Sterling you need a Pewter and a Pastel at the very least. A Pewter and a Super Pastel would increase you odds.
This is a Sterling Pastel (Super Pastel / Cinnamon) right after it emerged from the egg and before the first shed.
This is the same Sterling Pastel after it’s first shed.
This is the same Sterling Pastel as an adult and also a breeding picture.