On: Nvui: A NeoVim GUI written in C++ and Qt

[Lio]

Many terminals can display coloured undercurls that are different to the current text colour if that's what you mean.

E.g. here's a plugin for simple-term to add support for it: https://github.com/hexoctal/st-undercurl

The Kitty terminal has all kinds of graphics addons including coloured undercurls and images too:

https://sw.kovidgoyal.net/kitty/underlines/

Vim and NeoVim both support coloured undercurls too.