Put the white chocolate and white candy melts in a heatproof bowl.
Melt gently together. This is best done over a bain marie (also known as a double boiler). Place an inch of water in pan on the stove and bring to a gentle simmer. Sit the bowl on top so that it is not touching the water. Let the water simmer gently and stir the melts and chocolate as they are slowly melted by the heat from the steam.
Melt the blue and red candy melts in two separate bowls using the same method.
Spoon the white candy melts on a metal tray lined with baking paper. Spread gently to the edges of the paper.
Drizzle the red and blue candy melts on top. You can drizzle the melts off the end of a spoon, or put them into piping bags with the ends snipped off.
Swirl with a cocktail stick (or a skewer or the handle of a teaspoon).
Scatter the sprinkles over the top.
Dot the M&Ms on top.
Place in the refrigerator to set, then break or cut into pieces to serve.