That's because brown is the anti-neon.
Neon thrives on contrast, that's why so many things paired with neon (neon lights, glow-in-the-dark stuff, laser tag, raves) are on black or very dark blue. I think that principle makes neon work on black people, but they need to lean into the contrast. You can't just have dirty-ass Hot Cheeto Red weave in your grown-out braids and rock the look, that's ghetto. These looks off of pinterest are thought out, high contrast, and done well:
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(and on white models so I'm not cherry picking)
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Problem is, in animation and in real life, most things aren't contrasty enough to make neon work.
If they wanted to make a neon cartoon with brown people they should've made it as contrasty and stylish as Urbance or Spiderverse:
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The problem is they pussed out and didn't push anything hard enough. They do that all over in animation, they make everything blobby and safe and mediocre. If they actually pushed the colors, maybe like...
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Obviously it's still blobby brown lesbians, I'm not redrawing the whole scene, but it looks way better imo. The neon colors pop because there's a lot of dark to contrast it! Plus, darkening the scene and leaving the important elements (the starry sky, the main characters) bright makes them stand out while less important shit fades into the background. I don't know when "put more detail into the most important things and less into the less important things" became arcane knowledge, but here we are.