Preheat the oven to 180C/350F degrees.
Line up 6 small bowls and piping bags ready for the sponge colouring
Grease and line a rectangle baking tin ( 9 x 13 inch, 22.9cm x 33cm) with parchment paper.
In a medium bowl, whisk the egg whites until stiff peaks form.
In another medium bowl, beat the egg yolks, vanilla and monkfruit together.
Add the coconut flour, baking powder and salt and whisk well.
Fold in the whisked egg whites gently until the mixture is even in colour.
Spoon the cake mixture evenly into the 6 small bowls.
Add a couple of food colouring drops to each bowl to get the shade of colour you want.
Spoon each colour mixture into an icing bag (I sealed mine to stop the mixture running out).
Pipe each colour in the order of the colours of the rainbow into the cake tin,either length or width ways.
Bake for 15-20 minutes until firm.
Remove the baking tin from the oven and allow to cool for 5 minutes.
Put a large piece of parchment paper on your countertop and place the baking tin on the parchment paper, sponge facing down. Remove the tin from the sponge.
Using the parchment paper on the top of the sponge, gently roll up the cake. This is to keep the sponge in the correct shape before adding the filling.
Set aside to cool.