Since an electron is smaller than visible light, then what what color would a group of electrons (trillions of electrons) be if there were enough of them to be seen by the eye? What color would a group of trillions of protons be? Color of trillions of neutrons? I don't mean a group of electrons, protons, and neutrons mixed together into atoms, I mean a group of each of them separately. Would they be an actual color (red, black, green, etc), clear but visible (the color of water, glasses's lenses), or invisible?
There isn't a simple answer to that.
Colour arises when the light absorption or emission of a system is dependent on the wavelength. For example chlorophyll (i.e. plants) is green because it absorbs red and blue light so only the green light is reflected and reaches our eyes. So the question would be how does the light absorption and emission of trillions of electrons etc depend on wavelength?
The problem is that the light absorption and emission of electrons is dependent on their environment. For example an electron in a hydrogen atom absorbs and emits light differently to free electrons. This dependence on environment will also apply to protons and neutrons.
However, as a general rule a gas of charged particles is going to interact with light much as a metal does, so if you can make it dense enough a gas of electrons or protons will look silvery in reflected light. Neutrons aren't charged and their interaction with light is a lot weaker than electrons and protons so at similar densities a neutron gas would be transparent.
As for emission, electrons and protons will emit light due to black body radiation, so the colour of the emitted light will depend on temperature. As you heat the particle gas it will glow first red then yellow then white just as a heated metal does. Again neutrons are the odd one out since they have no charge. To a first approximation a neutron gas will not emit light.
The 'color' would be an ultra-bright burst of gamma rays as the trillions of electrons rush apart, frying both you and your eyes to a crisp.
More seriously, if you confined the cloud of electrons, it wouldn't emit any particular color on its own -- for instance, there could be no optical transitions since there are no nuclei.
If you shined light on it, it might behave like the electrons in the 'free electron model' of metals. Since the electron response determines the color, I guess it'd be a shiny grey, like typical metals. For more detail, see section 5 here.