For all we know, a single visible-light photon coming out of otherwise total darkness may not be enough to stimulate even a rod cell. A barely visible star puts some thousands of photons per second into our pupils. I don't know how many rods are involved in a sharply focused image.

If a single cone is stimulated, it tells us only that the light that stimulated it is somewhere in the visible range. All three types respond over most of the visible range, with a large overlap even between the blue and red ones. If I am not mistaken, our sensation of color is the result of the brain's comparison of the relative amounts of output from at least two of the cone types.