Instructions
Click the front door of your property after loading "Pet Society." Click the "Outdoor" store when the map appears.
Press the right arrow button on-screen at the Outdoor store to scroll across to the biscuit section. Find this section by looking for the sign above it stating "7 Amazing New Biscuits for One Incredible Price."
Click on the biscuits and purchase them. Go back to your Petling's house and click on the chest icon. Drag the biscuit that corresponds to the color you wish to change your Petling to into the room and then click on it. Your Pegasus Petling will eat the biscuit, and his color will change accordingly.