What substance gives many TNOs their distinctive reddish hue?

Complex organic compounds known as tholins

Many objects found in the far reaches of the solar system are covered in substances referred to as tholins. Tholins are complex organic compounds formed through the irradiation of simpler organic chemicals like methane by ultraviolet light and other forms of radiation. The presence of these compounds on the surface of these icy bodies is responsible for their characteristic reddish coloration, which provides astronomers with clues about the chemical environment and potential surface evolution of these distant worlds.