That’s because the icing is made from unicorn poop!

From a comment under the video:

The decorator ices the cake and uses a frosting comb to give it a wavy pattern, afterwards using a airbrush machine colors one direction in one color, avoiding to get some on the other side afterwards, they then, turn it on the blank side carefully add the second color, avoiding mixing the two colors. By having a texture on the cake, it makes it easier to do these effects!

[CharlotteSometimes | Via LS]