Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Line 2 x 12-hole muffin trays with paper cupcake cases.
In a mixing bowl, stir the cake mix, eggs, oil and water gently together, then whisk (by hand or electric mixer) for 2-3 minutes until smooth and creamy. Divide the mixture evenly between the cupcake cases. Bake both trays together in the oven for 18-22 minutes. Remove from the oven and transfer the cupcakes to a wire rack to cool completely.
Using a melon baller or teaspoon, scoop out the centre of each cupcake reserving each scooped out piece. Place a mini chocolate egg and a few sugar sprinkles inside each hollow (reserving some for decoration). Slice some of the cake off the bottom of the reserved pieces, then place on top of the filling, pressing down gently to give a level surface to each cupcake.
Divide the icing equally between 4 bowls and beat in enough of each food colouring paste or gel to give each icing a different pastel colour. Fit a large plain or fluted piping nozzle to a disposable piping bag, and spoon in one colour of icing. Pipe swirls or small blobs on top of 6 cupcakes. Repeat with the rest of the coloured icings, removing the nozzle from the bag each time, washing it and placing in a new disposable bag.
Sprinkle each cupcake with the reserved sugar sprinkles. Store any leftover cupcakes in an airtight container in the fridge for 2-3 days.