You get it by doing a level 55+ bounty, and speaking to the appropriate person who then rewards you with your own ark. As far as customisation, you can get different weapons and a few armour pieces for it, I've currently got the basic sword and a hammer, but am aware you can get a spear, another sword, a shield and I think a bow. Not too sure about changing the colour, there's a set colour, of 4 shades, as well as more colours in a list that are just question marks, which I believe are opened up as your GR goes up. Hope that helps!