The nameplate texture is part of ElvUI - if you don't use ElvUI, I reccomend changing the health bar texture to "Details flat" or "Grid flat" or similar depending on what addons you have installed. I'll add a better explanation in a future update.
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NOTE: This profile comes with NPC health bars colour-coded by default. Standalone colours: https://wago.io/c297bUudj
Many mobs have varied abilities and so cannot be described by just one colour, so the colours are guidelines to differentiate mobs at a glance and help with situational awareness.
Note in particular that mobs marked as urgent/priority are not necessarily to be killed first, this just suggest that they are particularly lethal or have dangerous abilities from multiple colour categories.
IMPORTANT: Make sure to uncheck "Target Shading" in the Target options, or the colours will not be clear in combat.
Colour
Description
Green
Boss.
Pale green
Mini-boss. Unique or almost unique mob with multiple abilities.
Magenta
Urgent/priority mob. This covers a range of priorities, for example: severe dps checks, extremely lethal abilities, mobs that must be kited as they have lethal auto-attacks, mobs with multiple dangerous abilities that don't fit into one category, "captain"-type mobs that reduce damage to mobs around them, etc.
Purple
Summoner. Summons more mobs, or transforms/evolves into a more dangerous mob.
Blue
Dodge. Casts multiple spells that require movement to avoid, or just one spell that is particularly dangerous or requires significant/frequent movement.
Red
AoE. Has a damage spell that hits the whole party, which either needs be stopped, or measures taken to survive the damage.
White
Healer. Heals/shields itself, or other mobs.
Cyan
Mage. Non-trivial interruptible damage ability/abilities.
Pale blue
Trivial mage. Similar to mage, but either cannot be interrupted (and not dangerous enough to fit into another category), or casts can typically be ignored.
Olive
Crowd control. Similar to mage, but casts crowd control instead of significant damage abilities.
Brown
Melee. Primarily tank mechanics, or the main danger is autoattacks. Frontal cleaves likely.
Grey
Trivial mob. Unimportant mob that will either not be pulled during the instance, or may be ignored and cleaved. Note that these may still deal significant damage to the tank on higher keys!