If you guys are planning on just using a rainbow highlighter for a one off event and don't want to pay $20+ for it or DIY it, here's a really easy way of doing it!
1. Pick your eyeshadows/pigments/micas of choice. If you're using a pressed eyeshadow, scrub hard until you create a lot of powder.
2. Collect the fall out on your brush
3. Brush it onto your flat, stippling brush in a rainbow layer
4. Swipe once on your face. That's it!