Tint the chocolate frosting as dark as possible using black icing gel color (I prefer Americolor brand). Transfer the frosting to a pastry bag fitted with a large round tip (such as Ateco #809). Pipe a generous swirl on top of each cupcake, and use a small offset spatula to smooth into a mound, being sure to fully cover the edges of the cakes in black. Place the black sanding sugar in a bowl and very gently dip the frosted portion of each cupcake into the bowl to adhere a thin layer over the entire surface without distorting the shape. Once fully coated, you can gently cup the frosting with your hand to help mold it into an even mound.
Use the black edible marker to draw pupils onto the colored candies. (If the candy you are using has any markings, be sure to draw on the blank side.) Pair up the eyeballs by color and gently press two to three pairs into the frosting of each cupcake.
*Use the best quality, greaseproof black liners you can find. If they are thin or flimsy, the cake will show through and lessen the effect of the blackness contrasted with the colored eyes. See the FAQ page for suggested resources for good cupcake liners.