In the most recent developer blog, Phoenix Labs lays the foundation for the ways in which players can customization their slayer.
In a word, dyes. Dyes for days. And not just dyes, but also the ability to chose a type of finish (matte, glossy, etc). There are also rare cosmetic items and a way in which for players to transmogrify one item to look like another. So you can take your awesomely statted helm and make it look like a badass one you just found. Or you can make it look like you only just started playing. It’s all up to you.
On top of color customization, Dauntless also features flares that you can use to show some flair. There will be several dozen of those at launch. Players will also have a banner that they can customize to show off their accomplishments.
Cosmetic items can be found by killing bosses and completing quests, and on top of that, Chroma Cores can be purchased through the in-game store. Those offer players a bundle of cosmetics.
You can check out the blog post yourself for a full scoop on character customization.