Highlightr-Plugin is superb! Thank you for linking to it.
I found the default colors overly saturated, and while numerically balanced appeared visually more in-and-out than I like. I have customized mine to be easier on my eyes and visually in the page with
Pink
FFB8EB80
Red
F0558260
Orange
FFB86C70
Yellow
FFF3A3A0
Green
BBFABB70
Cyan
ABF7F766
Blue
ADCCFF80
Purple
D2B3FF80
Grey
CACFD9A6
Color perception curves vary by individual, so YMMV — I hope that provides a better place to start than the defaults for anyone who wants quieter highlighting. Done on a hardware calibrated monitor. I use the equally superb Minimal Theme, in Light Mode, with Light Mode Style set to “Low contrast”.
I also recommend following the directions given by the author on the page linked by @lavaldi and assigning custom keychords to apply regularly used highlights.