Userscripts.

This where you can find cool userscripts for Neocities!
NOTE: You need a userscript plugin to use these, like Tampermonkey.

Hex Color Preview

It displays the color of a hex code as a small outlined square positioned over the pound sign, making it easier to visualize and manage color codes in their projects.

It will, however, not help you preview hex codes with an alpha channel. Figure it out in your head.