Some existing examples are "warning squiggles" added by IDE integrated linters, as well as language-level syntactic markers for "here be dragons" parts like Rust’s `unsafe {}` that can be highlighted by standard syntax highlighters (as an aside, it occurs to me that for simplicity’s sake most syntax coloring is in fact merely lexical coloring, but I digress…)

It seems to me that many color schemes these days choose to render comments in grayed-out, low-contrast colors. I find this to defeat the purpose of comments and maintain the unfortunate notion that they are something nearly redundant. A properly written comment tells me something nonobvious and should be displayed prominently rather than faded into obscurity!