In addition to customizing armor, you are allowed to customize the color of your visor from the traditional yellow gold to a variety of colors. There are 5 colors available to choose from. These are purely cosmetic and have no effect on gameplay whatsoever. The colors available are Default, Silver, Blue, Black and Gold.
Visor colors are seen again in Halo 4 but with a wider variety to choose from. Visors can be unlocked by leveling up, completing Commendations and completing Specializations. Like Halo: Reach, these are purely cosmetic and have no effect on gameplay.