This pen pairs well with paper, though it can also go with skin (your own, preferably) or any other material that can be written on. The beauty of an accessory like this is that even if you mix your metals, it's going to look good.
Over time, the pen will patina and develop a look that's uniquely your own. This is totally normal and shouldn't change the way you use it at all. If you want, a normal brass cleaner will help remove some of this patina and restore the shine. See the second image for a pen that has developed a patina over a year of usage.