In my experience, the biggest wins in syntax highlighting come from just a few wins: make comments a different color, make strings a different color, and if you've got something like shell where strings can contain embedded variable references, pop those variable references into a different color.
One of the big problems with a lot of the examples here is that, well, I spend most of my time on multimillion line codebases. If you want to pop stuff out to me, showing it in a different color is useless because it's not on the same screen; no, the way you give it to me is a macro that takes me to the next location of the thing of interest. And with a macro that lets me move to points of interest... the use of color is entirely redundant.
I've just installed a plugin in my emacs config the other day called dumb-jump[0]. The way it works is by running a grep tool (grep, git-grep, the_silver_search (ag), ripgrep) inside the current project with some patterns according to the file type. It hooks into emacs xref (dynamic links and history).
These days, I'm using very minimal highlighting (doric-themes [1] which is basically shade of one color and font-weight). I prefer to separate semantic units of code by whitespace. Then scanning becomes quick.
I 80% agree with this, but I've also gotten a lot of mileage out of Emacs hi-lock mode. The simplest thing I always want: whatever symbol I'm pointing at right now, highlight everywhere (so I can trace it through the code quickly); more generally, I want to highlight an arbitrary regex, so for instance when I'm auditing a codebase for authorization, I quickly spot the functions that aren't following the same authorization patterns as all the others.
One of the big problems with a lot of the examples here is that, well, I spend most of my time on multimillion line codebases. If you want to pop stuff out to me, showing it in a different color is useless because it's not on the same screen; no, the way you give it to me is a macro that takes me to the next location of the thing of interest. And with a macro that lets me move to points of interest... the use of color is entirely redundant.