In a large mixing bowl, whisk the egg whites and cream of tartar together until frothy.
Gradually add the sifted powdered sugar to the egg whites, mixing on low speed until fully combined and smooth.
Add the vanilla extract and continue to beat the mixture on medium speed until stiff peaks form, about 5 to 7 minutes.
Divide the royal icing into separate small bowls, depending on how many colors you want to use.
Add gel food coloring to each bowl and stir well until the desired color is achieved.
Adjust the consistency of the icing by adding a few drops of water to thin it for flooding or adding more powdered sugar to thicken it for piping details.
Transfer the icing to piping bags fitted with small round tips or use squeeze bottles for easier control.
Outline your sugar cookies with thicker icing and let it dry for 10 minutes.
Flood the inside of the outlines with thinner icing and use a toothpick to spread it evenly if needed.
Allow the decorated cookies to dry completely at room temperature for at least 4 hours or overnight before storing or serving.