Once I had all my frosting bags ready, I did each color, then bounced back and forth between the various sizes and colors, filling in holes to finish up. As long as you keep a good spread, you can’t mess it up. It’s just a random coloring process, which is what makes it so easy. Just don’t clump a lot of one color together in big groups, and you’ll do just fine.
It’s super humid where I live, so I paused about halfway through decorating and placed the cake in the fridge to let the frosting firm back up before continuing. If need be, this is a great way to keep the cake looking perfect at serving time instead of partially melted. If your frosting bags during the decorating process start to get super squishy and the frosting starts acting super soft, go ahead and let it go in the fridge for a few minutes to firm back up. It makes for nicer decorating. However, note that if the frosting is too cold, it won’t come out of the tips.
You can use any combination of colors you prefer. I would just at least use 4 or more colors to really give it that colorful look.