What colour is Pluto?
Short answer: Reddy-brown.
Long answer: Before 2015 we didn't have close-up photos of Pluto and it was hard to tell exactly what colours we might find there. We did know that Pluto is generally reddish but we were very vague on the details.
When the robotic probe New Horizons flew past Pluto in 2015 it took enough photos to give us a good look the dwarf planet's colours. It turned out that Pluto is mostly shades of reddish brown. The redness is probably caused by hydrocarbon molecules. These are formed when cosmic rays and UV sunlight interact with methane in Pluto's atmosphere.