system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do neon colors work?

What makes them "pop" more than regular colors? And why can't I imagine a neon brown?
gpt
Sometimes instead of being called neon colors they are called "fluorescent" colors.  What makes them fluorescent is that they absorb light you normally can't see (UV light) and then release that energy as visible light.  The result is that if you shine light that has UV in it (like sunlight or a blacklight) instead of just reflecting light like normal colors do, they actually glow as if they were a weak light bulb.  The end result is that there is actually more light of those colors hitting your eyes so they seem brighter.   

As for why you can't think of "dark" neon colors, it's because what makes them distinct is that they add more light, while darkness is actually the absence of light.