Preheat the oven to 180 degrees C (350 degrees F). Line a shallow tin (20 x 30 cm) with baking paper, using a little butter to make sure the baking paper sticks to the tin.
Whisk three eggs and 95 g caster sugar together using a hand mixer intil light and fluffy. Then add 2 tablespoons of warm water and 1 tsp vanilla extract.
Sift in 95 g plain flour, a bit at a time. Fold it into the cake mixture.
Take three dessertspoonfuls of the cake mixture and put each in a separate small bowl. Colour each bowl using professional cake colouring paste to the desired shade. I made red, green and blue.
Spoon each colour into a separate small piping bag and pipe coloured dots onto the baking paper lining the tin, in the desired pattern.
Put the tray with the dots in the fridge for ten minutes.
After ten minutes spoon the rest of the cake batter over the dots and smooth the top.
Bake the Swiss roll for 14 minutes. Look for the edges shrinking away from the sides of the tin to check when it is baked.
Remove it from the oven, and turn the cake out onto another pieces of baking paper with caster sugar sprinkled on top.
Peel off the backing paper to reveal the dots, then turn the cake back over so the dots are underneath.
Next roll the cake up, with the short side towards you. Wrap it in a clean tea towel or piece of baking paper so it remains in the roll shape as it cools.
Once fully cooled, unroll the tea towel or paper and the cake inside, spread jam over the inside, then roll the Swiss roll back up tightly and dust with icing sugar.