Though royal blue food colouring is available already made, the shade it produces often isn't the exact one that the baker wants. You can adjust the shade using two other colours. Buttercream icing is a great way to test out the dying abilities of dyes as it holds colour well and is easy to handle. Bear in mind that dyed icing does darken as it sets so you'll want to make it a bit lighter than you really want.
Place 1/2 cup of prepared buttercream icing in a small bowl. Add in drops of royal blue food colouring until you've reached 10 drops. Stir vigorously and place the icing in the refrigerator for an hour.
Look at the colour that the icing has settled into. Adjust the colour recipe for the shade of royal blue you'd like. Try adding drops of red food colour to brighten and drops of purple food colouring to deepen the hue. Start over with another 1/2 cup of icing if you want to start from scratch. Add in white icing to the dyed icing to lighten it.
Mark down the number of drops per shade that you used for the 1/2 cup of icing. Adjust the number of drops according to how much icing is needed for your project; for example, four cups of icing will require twice as much of each drop.