You can achieve really nice looking syntax highlighting using only bold, _italic_ and several different scales of gray.

Differentiating between various grays seems like a waste of time when you could just use color

you could also say using color seems like a waste of time when you could just vary letterforms. e-ink is nicer for your eyes but expensive if you want color. how one goes about highlighting their code depends on a number of constraints; if you want a display that's easier on the eyes you might use methods other than color to highlight your code. it's not easier or harder or any more or less natural or anything.

me personally, i've always used color vary sparingly, if at all, because a bunch of highly contrasting colors ends up looking, to my eye, like vomit soup without really clarifying anything.