We suggest that you tint all icing at one time while cake cools. Refrigerate tinted icing in covered containers until ready to use. Bring to room temperature for decorating.
Make 4 3/4 cups buttercream icing:
Tint 3 cups light royal blue (thin 2 cups with 2 teaspoons light corn syrup)
Tint 1/4 cup royal blue
Tint 1 1/4 cups black
Tint 1/4 cup gray