I recently bought some rainbow filament to try out, mainly because I wanted to print some vases and get a cool color gradient effect. And it totally works, as long as you're using over 10 meters of filament. My first print came out in one solid color as it only used almost 9 meters of filament. But the three print above all used somewhere between 12 & 15 meters of filament which provided some nice color changes.
I used Silk Multcolored PLA filament from Amazon and printed these at .20mm layer height at 215 C for the first layer, 210 C the rest, bed temp at 60 C on my Prusa I3 MK2.5. And if you're interested in these vases, you can check them out on Thingiverse:
Curved Honeycomb Vase
Spiral Vase
Spiral Vase
This is definitely the shiniest filament I've used and I have some silk gold PLA as well that I'm going to try out next so stay tuned!