At http://earthsky.org/space/pluto-paints-its-largest-moon-red … Images of Chiron beamed back to earth showed the moon of Pluto was reddish in colour in its northern parts. There was speculation at the time on why this might be and now an explanation has been offered – the red colour was painted by nearby Pluto. The theory is that methane gas escaping from Pluto's atmosphere became trapped by Chiron's gravity. Subsequently, the squirted gas froze at Chiron's north pole and a chemical process occurred in which ultra violet light from the sun transformed the methane into heavier hydrocarbons (and eventually into a reddish organic molecules known as tholins).
The article is in Nature and the blog author at Earth Sky accepts the findings without any questioning. Needless to say the results were obtained by modelling. This began with the premise that methane was involved. The modelling also requires the sun to convert the methane into other hydrocarbons and to obtain this in their model they used the 248 year orbit of Pluto (and Chiron) to show that for 100 years there was constant sunshine (warmth) followed by 100 years of darkness (cold).