Piping the mixture
Preheat the oven to gas 6, 200°C, fan 180°C and line a 25 x 35cm baking tray with nonstick baking paper. Put the sugar and eggs in a large bowl and beat with an electric whisk on medium-high for 10 mins until thick and airy. Divide the mixture evenly into 3 bowls. Add the pink food colour gel to one portion and the blue food colour gel to another; gently fold into the mixture with a spatula until no specks of gel can be seen. Leave the third bowl uncoloured.
Sieve 3 tbsp flour over each of the 3 mixtures. Gently fold in the flour, cleaning your spatula between each bowl.
Transfer the mixes into 3 piping bags fitted with a large round nozzle, or cut to make a 1cm wide opening. Pipe a line of plain mix diagonally from one corner of the tray to the other. Sandwich this with a line of pink mixture either side, then sandwich the pink mixture with a line of blue. Repeat using the pink, then plain, back to pink, and blue until the tray is covered. If you have any left over, you can top up the lines you’ve already piped.