NASA releases this photo which shows Pluto’s reddish color and Charon’s gray tone. Scientists think Pluto’s red color is the result of particles created in its atmosphere, through methane’s interaction with UV light. The particles stick together, growing heavier, and eventually rain down on the surface. Observations show that Charon does not have much of an atmosphere.
For now, all that we can say is, it’s a much more rarefied atmosphere [than Pluto’s]. It may be that there’s a thin nitrogen layer in the atmosphere, or methane, or some other constituent. But it must be very tenuous compared to Pluto — again, emphasizing just how different these two objects are despite their close association in space.