I found color changing nail polish at Del Sol at Sea Port Village in San Diego, CA while on vacation. They had a buy 3 get one free special going on with each priced at $10.
Featuring the color “So Good”. Left: inside; Right: outside window for 30 seconds on a cloudy day
Here’s a picture of it in action in direct sunlight. It creates almost a gradient effect as it’s transitioning. It goes from yellow/white to peachy orange. Apparently it’s a special crystal technology that allows the polish to change colors in sun light (UV light). They also sell clothing and accessories that change color too! Bonus, the plastic bag upon purchase changes color too! Here’s the website.
The color was a little opaque and I had to apply 3-4 coats to get a solid color. Also, I find it transitions faster and more vibrant without a top coat. They do sell their own top coat so maybe that will work better but a regular top coat inhibits the color change. The more direct and bright the sunlight, the more vibrant the color change. I was a little skeptical whether it would actually work but lo and behold, it does! It is a little pricey but it’s pretty revolutionary.