Make the royal icing as directed in the linked recipe, adding the orange extract in place of the vanilla extract.
Place ⅓ of the royal icing into a small bowl. With a toothpick, add 1 or 2 drops of the green gel food colouring or until desired green colour is reached. Stir to evenly distribute the food colouring throughout the icing. Place half of the green icing into a piping bag fitted with a #2 or #3 round/plain piping tip.
Add a small amount of water, about a ½ to 1 teaspoon at a time, to slightly thin out the remaining icing in the bowl; it should have the consistency of honey. If it gets too thin, add more icing sugar. This will be used to flood the leaf part of the cookie. Place the thinner icing into a piping bag fitted with a #3 or #4 piping tip.
Add 2 to 3 drops of orange gel food colouring, or until desired orange colour is reached, to the remaining ⅔ of the icing left in the bowl. Remove half of the orange icing and place it into a piping bag fitted with a #2 or #3 round/plain piping tip.
Add a small amount of water, about ½ to 1 teaspoon, in the bowl until the remaining orange icing is the consistency of honey. This will be used to flood the orange part of the cookie. Place this icing into a piping bag fitted with a #3 or #4 piping tip.
With the green icing (flooding consistency), pipe an outline and fill the shape of a leaf onto the upper part of the cookie using part of the upper contour of the cookie as one side of the leaf. Repeat process with all the cookies. Going back to the first cookie, outline the remaining edge of the cookie with orange and flood with the orange flooding consistency icing. Repeat process with the remaining cookies.
Allow cookies to dry for at least 20 minutes before adding the finishing details. Outline the leaves with the green icing (thicker icing with the piping tip #2 or #3) and add veins to the leaves, if desired. With the orange icing (thicker icing with piping tip #2 or #3), add a couple of small dots, a few smaller than others, along one side of the orange to mimic the dimpling of the skin of an orange.
Place the decorated shortbread cookies back onto the wire cooling rack to dry completely; 6 hours or overnight.