In order to explain the colour of an atom or molecule⁺, one considers the orbitals of the electrons surrounding it and the respective energy level differences. A single free electron does however not possess any "self-orbital", and thus no colour in this sense. But let's consider higher incoming photon energies - starting with 511 keV, an electron-positron pair can be temporarily created, so there is some interaction with electromagnetic radiation. Of course, 511 keV is far beyond what the human eye can perceive, so "colour" must be generalized into something like "spectral scattering cross section", and thus the title's question more correctly is:
Considering (non-linear) QED effects, what is the scattering spectrum of a single isolated electron in its rest-frame?
Of course, coming back to colour, one might then consider the question:
Given that spectrum, what is its lowest-but-non-trivial energy limit, and what (angle-dependent) colour does it provide when convoluted with sunlight (i.e. black body radiation at ~6000K) and the CIE standard observer, neglecting the under-saturation?
⁺ or rather, its electromagnetic spectral resonances