I asked a slightly different question before, but how common are one color frequency items? Many things can reflect many colors of light, but predominantly show one under white light. If they are exposed to different colored light, they produce different colors, but this happens to many things, even black items. Are there any one frequency items that only reflect that color and become black with different colored light? How common are these items?
I'm going to agree with CuriousOne, but for a slightly different reason. If you want a single frequency - like, 400.000000..... nm (infinite precision, whatever that means), you must be talking about an atomic transition. Such transitions have "single wavelengths" because when an electron jumps from one state to another, that's a well-defined energy. That single (quantized) energy corresponds to a single wavelength of light.
So, why not make an object out of an element with 1 such transition, to make an object with a single color? Two major problems:
a) You will never observe an exactly 400.00000.... nm photon. Not only experimental errors, but the uncertainly principle ensures that even the photon itself has some spread over the wavelength. Maybe you can define "single color" as "wavelength +/- 0.001%", so that would be fine, but you'll never get "exactly" one color.
b) Any element with a "single wavelength" as I've defined it in (a) would necessarily have others. Even the simplest atom (Hydrogen) has many many energy levels between 3.5 eV and 13.6 eV - any real object will have many, many discrete colors, even if we are only talking about atomic transitions.
So, No, there are no single-frequency objects.