Wash and thoroughly scrub your beet. Then finely grate over a paper towel or thin, clean dish towel, being sure to set the towel over a plate so the beet juice doesn't stain your countertops. 1 beet should yield roughly 3 Tbsp (45 ml) beet juice.
Once your beet is grated, gently squeeze the beet juice into whatever you're dying - in this case, it was buttercream frosting for vegan sugar cookies.
The more juice you add, the more intense the hue (see photos).
Other readers have suggested turmeric for yellow, pomegranate for red, and matcha powder or spirulina for green!