Surface colors vary from white carbon monoxide frost (the bright area in the middle image), to dark orange and charcoal black regions in which ultraviolet radiation from the Sun is presumably breaking down methane ice molecules, creating a carbon-rich molasses-colored sludge, and lighter gases which quickly freeze, to create the brighter regions.
Interesting! I got my information from the links below.
http://i.space.com/images/i/000/007/793/i02/ig390-hubble-16-02.jpg?1296061825
http://cseligman.com/text/planets/pluto.htm