if you were able to "zoom in" far enough to see the elementary particles, what color would they be ?
They have no intrinsic color. Color is a result of photons that your eye detects and to "see" an object it has to radiate or reflect light.
It's complicated but to "see" something photons have to interact with the thing you're seeing. There's no single way that happens for an elementary particle and hence no one appearance for any particular particle.
When you see everyday colors they are almost all due to the absorption and emission of photons when an electron changes energy level in an atom. But while electrons are elementary particles, the energy levels and their differences are a property of the atom as a whole, not the electron itself. The electron has no color itself.
And if they don't have a color, what would they look like?
What they would look like is a bit meaningless in relation to an elementary particle. You cannot even localize the position of an elementary particles (the uncertainty principle at work) and so it has no definite position, let alone a definite color.
There's another issue. In quantum theory when you observe something (make any measurement or interact with it at all) you change it's state. So even if you observe a photon after it has interacted with an electron, the electron itself could be in any state (again, the uncertainty principle working).
So there is no color and no meaningful human way to describe what an elementary particle looks like.
Which is why we use quantum theory to describe the behavior and properties of these things : nothing else makes sense except the mathematics.
