For each piping bag, use a large nozzle- it doesn't matter which shape they are, as the applied buttercream will be smoothed afterwards. Fill your piping bags: one for each colour. Using your first bag, pipe the corner of the cake to form a triangle. You want to aim for around an inch width for each colour.Use the piped triangle as a guide for where to start your next colour. Using your second colour, form a large stripe next to the triangle making sure it touches. Continue to do this, alternating all your colours until your cake is covered with buttercream stripes.