Remove the cake from the pan and leave it on the parchment paper. Using the colored buttercream, spread stripes of the frosting lengthwise on the cake. Using parchment to lift and push forward, roll the cake lengthwise as tightly as possible. Once the cake is rolled, place on a large platter, seam side down. Shortly before serving, frost the cake with the whipped cream. Spread the whipped cream on generously and create swirls using a palette knife. To make the cloud-like swirls, lightly drag the palette knife through the whipped cream and gently pull up to make peaks. Decorate the platter outside of the cake with strawberry coulis and sprinkle the whole thing with confetti sprinkles. As the cake is sliced, a "rainbow" of frosting will be revealed on the inside.
Combine the strawberries, water, sugar and lemon juice in a saucepan and cook over medium heat until the sugar is dissolved and the strawberries are quite soft. Puree the mixture in a blender. Strain through a fine strainer and cool.