Use untinted icing to make the Xs and Os to decorate this XOXO wedding cake (reserve 2 tablespoons icing to use as a glue; refrigerate it airtight). Tint the second batch different hues of red to serve alongside each slice.
Beat all ingredients in a mixer bowl on low until smooth and thick, about 15 minutes. If desired, add food coloring, a little at a time, to achieve the desired shade. Icing should be stiff enough to hold its shape but still pipable; if it's too thick, add water by the teaspoonful; if it's too thin, beat 2 to 3 minutes more (for the Xs and Os a thicker royal icing is desired).
Line a baking sheet with parchment; draw rectangles on paper (2 by 1 1/2 inches for Os, 2 by 1 1/4 inches for Xs), then flip. Place icing in a pastry bag with a basket-weave tip (such as Ateco #45). Pipe Xs and Os (Xs 2 inches high by 1 1/4 inches wide; Os 2 inches high by 1 1/2 inches wide) onto prepared sheet in rectangles. Let set until hard, at least overnight. Store airtight up to 4 weeks.