I use some custom target arrows yoinked from Naowh, download the addon "SharedMedia" and unzip this folder into your addon folder: https://drive.google.com/file/d/1tIL7UHWl3ipVbGyt9D9xcMEF8k6TFN4g/view?usp=sharing
The colors don't have a HARD set of rules I use, but in GENERAL it's
orange = big guy in a pack or frontal/cleaves
brown = another orange if orange is used, or annoying tank mobs
cyan = primary kick or heal kick
dodgerblue = another kick
orchid = annoying mob or one that has something mobility related (jump out)
fuchsia = certain important mobs (such as some affix mobs, lieutenants, minibosses, etc)
white = high priority targets and stuff
dimgrey = small add, sometimes completely irrelevant or sometimes just less important
feel free to change them or add more in the "Npc Colors" tab
If you are doing solo content like Mage Tower or something, disable the Mod called "Force Threat Color NPC" in the Modding tab so the nameplate colors work properly.
If you want to make more mob spellcasts glow their castbar then add the spell ID as a trigger in the script called "M+ Important Spells [Essoh Edit]"