Preheat the oven to 350 degrees.
Spray a 13x18 baking sheet with baking spray and then line it with parchment paper. Set aside.
Using a standing mixer with the whisk attachment, beat the egg whites, lemon juice, and salt until the mixture becomes foamy.
Gradually whisk in the sugar.
Continue to beat until the mixture forms soft peaks.
Gently fold in the cake flour until combined and smooth.
Divide the cake batter into 4 bowls. Then mix in a few drops of food coloring (one ofr each bowl) until you reach your desired shade.
Alternate between all 4 colors, dropping dollops of the batter onto the prepared baking sheet.
Using a flat spatula, spread the batter into an even layer.
Bake in the oven for 12 minutes or until it springs back when gently pressed with your finger.
Remove and let the cake cool for 5 minutes before removing it from the pan.
To remove from the pan - place a piece of parchment paper on top of the baked cake.
Then take another baking sheet that is the same size as the one that you baked the cake. Place it on top of the parchment.
Then gently hold the two sheet pans together and flip the pans over.
Gently pull up the one pan on top - this is the one the cake was in. It should release and the cake should now be on top of the back side of the other pan.
Remove the parchment paper, lay a lint-free towel over the top of the cake, and then tightly roll the cake into a spiral.
Place the rolled cake into the fridge and allow it to cool for a few hours.