Navigate to the Storefront - Product Manager.
Click Configure - Colors.
Select Add Color to enter in a color, then Save to save your changes and add the color to your list. The list you create here will appear as a drop-down menu when selecting a color variation for your products, if applicable. The color selector will not appear if your product is not configured to have color variations.
If you were already using the Storefront module prior to this enhancement and have selected colors for your products, those colors will automatically be added for you.