You can visit a dye station and interact with it to dye any piece of armor that you own.
Only armor can be dyed.
Dye stations can be found in all major cities (With the exception of Davon's Watch). There are special markers for these dye station locations
Every piece of gear has up to three separate areas that can be dyed individually, and you have tools available to do so.
Color Sets: You can use the special dye tools to create and apply custom color sets.
Eye Dropper Tool: You can use the eye dropper tool to pick colors from previously dyed gear, and apply that color elsewhere.
Paint Bucket Tool: This tool applies the dye you’ve selected to the corresponding color area of every piece of gear you have equipped.
At a dye station, you can see every color available. Dyes you haven't obtained yet are displayed with a lock icon. Hover over any locked color to see which achievement is associated with it. You can right-click on it to jump to the achievement panel and read the achievement's completion criteria. When a dye is unlocked, it will be available for all characters on your account.
There are well over 200 dyes to unlock and use.
You are able to choose from the following hues, each with different shades within them: