There are huge, huge amounts of customization possibilities for your armor and appearance. With over 400 different full sets of armor across the four Tiers, not to mention one-off item models, you’re going to see a lot of variety on the battlefield.
Most pieces of armor can be dyed in two locations, generally one main color and one secondary color for the accents or second major area of the armor. For instance, a chestpiece with a tabard over it would typically give you the option to dye the tabard separately from the main piece of armor, with whichever colors you’d like. Dyes come in many different colors, ranging from inexpensive ones from vendors to rarer drops from monsters. Vendors offer a minimum of 15-20 different colors, with a larger total number than that in the game. When you go to a merchant that offers dyes, you can select an option to be presented with the Dye Items dialog, shown below.
As you can see, there is an area with swatches of each of the colors, and your equipment shown with your character standing in the middle. When you hover over a color with your cursor, it tells you its name and cost. You can then click on one of your pieces of armor, and if it is able to be dyed (almost all are except for your Rank 1 starter armor), you will then be able to left click on a color swatch to set the primary color, and right-click to set the secondary color, with the character showing a preview of what it would look like. Once you’ve decided if you like it or not, you can “OK” the selection, or cancel it. After deciding everything you want to dye is “OK,” you can see the total cost of the dyes you’ve selected as well as how much money you have in the dialog. At this point you can accept the whole dye job, or cancel it if you don’t like the new look. It’s a very good customization system that works well in actual use.
Finally, Apothecary crafters can make special dyes with drops and other items, that work the same way other than right-clicking the dye bottle in your inventory, then selecting the armor you’d like to see it applied to (you’re then asked to “OK” or decline the change). These dyes range from being easy to get the ingredients for to very rare or difficult. This provides a way to find special dye components to really make yourself stand out with a more unique color than everyone else has!