In the second picture posted, note they say "enhanced" colors. These are not the true colors - they are stretched colors, with image processing, to show more subtle variations from within a smaller range of color. Pluto is mostly yellows and reds, with blue haze.
There are also methane (CH4), nitrogen (N2), and carbon monoxide (CO) ices on the surface. The MVIC camera mentioned has 4 measurement channels: Near IR, CH4 (methane), Red, and Blue. This article talks about the methane found:
Moore et al., 2018. Bladed Terrain on Pluto: Possible Origins and Evolution: https://doi.org/10.1016/j.icarus.2017.08.031
From another abstract about ices:
"The spatial distribution of volatile ices is compared with the distribution of the coloring agent(s) on Pluto's surface. The correlation of ice abundance and the degree of color (ranging from yellow to orange to dark red) is consistent with the presence of tholins, which are refractory organic solids of complex structure and high molecular weight, with colors consistent with those observed on Pluto. Tholins are readily synthesized in the laboratory by energetic processing of mixtures of the ices (N2, CH4, CO) known on Pluto's surface."
- Cruikshank et al: http://adsabs.harvard.edu/abs/2015DPS....4710102C
"Laboratory experiments suggest that Pluto's coloration may be due to the presence on the surface of tholin-like materials, which are organic compounds produced through photolysis and radiolysis of native material found on the surface and atmosphere of the dwarf planet. Tholins produced in the laboratory exhibit a wide range of colors" - Protopapa et al: http://adsabs.harvard.edu/abs/2018DPS....5050601P
There is also latitude variation:
"These observations reveal a large equatorial basin (informally named Sptunik Planitia), filled with N2 ice with minor amounts of CO and CH4, surrounded by hills of CH4 and H2O ice. Broadly speaking, composition outside of Sptunik Planitia follows latitudinal banding, with dark, mainly volatile free terrains near the equator, with N2, CO, and CH4 at mid-northern latitudes, and mainly CH4 at high northern latitudes." - Young et al: http://adsabs.harvard.edu/abs/2017AGUFM.P13F..02Y
And one not behind a paywall: Surface compositions across Pluto and Charon: Grundy et al: https://science.sciencemag.org/content/351/6279/aad9189
Some more references:
This article talks about the material found in the haze that was seen around Pluto: Grundy, W. M., et al., 2018. Pluto's haze as a surface material. Icarus 314, 232-245. https://doi.org/10.1016/j.icarus.2018.05.019 - although I can't find it outside a paywall. It says the blue haze is mostly molecular nitrogen, with smaller amounts of methane, acetylene, ethylene and ethane.
The other responder noted Titan haze, everything you ever wanted to read about haze on Titan can be found here: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JE005240
It talks in much more detail about tholins.
As a side note, most of the authors are from various institutions such as NASA AMES, SETI, SwRI, and U of Arizona, not Harvard. Harvard just provides an abstract listing service for astronomy papers that have been submitted but not published yet, once you post, it's on that site forever. http://adsabs.harvard.edu/