The color code for Pluto: nitrogen ice is white. Where methane is exposed to uv and charged particles, it turns into tholins - dark red-brown hydrocarbon compounds. A little methane, peeking through the nitrogen ice, gives you pink terrain. Where there is no ice, accumulated dark tholins are what you see. Charon, which is looking dark gray, probably has a surface that is mostly old water ice.
Neptune's moon Triton is believed to be similar to Pluto and is likely a captured Kuiper Belt object:
Triton: hatched from the same region as Pluto. Voyager image.
Pluto and Charon in color: LORRI + MVIC, June 25 & 27, 2015
Pluto shows two remarkably different sides in these color images of the planet and its largest moon Charon taken by New Horizons on June 25 and June 27. The images were made from black-and-white images combined with lower-resolution color data. The left image shows the side of Pluto that will be seen at highest resolution when New Horizons makes its close approach on July 14. The hemisphere is dominated by a very dark region that extends along the equator. The right image is of the side that faces Charon; the most dramatic feature on this side of Pluto is a row of dark spots arranged along the equator. (The equator appears near the bottom of the images of both Pluto and Charon, as New Horizons' view is mostly of their northern hemispheres.) (From Emily Lakdawalla's blog).
Pluto, LORRI images, June 22 - July 1, 2015
Charon, LORRI images, June 22 - July 1, 2015