Guild Wars 2 Add-ons?
I don’t know exact links but look up Overwolf (Overwulf?) api GW2 and you should find some overlay type addons that are approved by ANet.
For the most part anything that alters the game’s design is not allowed. If it gets into the code, it’s no good.
Also you can check out SweetFX to alter image tones, I don’t know if that will change colors for you or not.
ones that would change the color and textures of the HUD and other such things, or are those none-existent?
That was done by manipulating the textures, which can also be done in GW2 in the same way. It’s technically against the rules, but they allowed it in GW1.
They have been clear that anything that interacts with the client is against the ToS. Anything that is an overlay that does not interact with the client is fine.