Preheat the oven to 180°C (160°C fan forced). Grease a 20cm x 30cm rectangular cake pan and line the base and sides with baking paper. Let the paper overhang the sides to make removal easier.
Open the brownie boxes, remove the enclosed frosting sachets and reserve for another use. Combine the brownie mix, butter and eggs together in a large bowl until well blended. Spread the mixture evenly in the prepared cake pan and bake for 35-40 minutes or until a round bladed knife inserted into the centre comes out almost clean. Remove from the oven and leave to cool in the pan for 10 minutes. Use the baking paper to lift the brownie out of the pan and onto a wire rack. Leave to cool completely; about 1 hour.
Transfer the brownie to a chopping board and cut out egg shapes using a 7.5cm x 5.5cm egg shaped cookie cutter. Gently push the brownie out of the cutter each time. (Reserve the leftover brownie pieces for another use.) If the cutter gets sticky, wipe clean or brush with a little oil to ensure a clean cut.
Divide the vanilla frosting between 3 small bowls and use the food colouring paste or gel to colour the frosting green, pink and yellow. Spread 1 tablespoon of the frosting onto each brownie egg. Decorate as desired with sprinkles and coloured sugar balls.
Leave to set for 30 minutes before serving or store, covered, in an airtight container in a single layer for up to 3 days.