Now, we do have equipment to generate single photon at a time, and LASERs are nearly monochromatic.
While typing the question, am realizing that successive photons in case of single photon generation may have slightly different wavelengths, but are its single photons essentially monochromatic? Because we may be able to use the phenomena to make highly coherent lasers.
Any measurement of the photon's energy (i.e. frequency, or free-space wavelength though making a direct identification of particle properties to wave properties is a little sketchy) will return a single value. Every time.
But ... you can't fool Heisenberg and if you have confined the position of the photons---say by insisting that it hit the detector---then
Yes - but not really.
A single photon has a single value for it's related wavelength - so it's perfectly monochromatic, in a formal sense.
But, in a physical sense, the concept of a spectrum (like: monochromatic) basically does not apply to a single photon.
In practice, you do not have these "single photons, with some specific wavelength" to actively work with, as implied in the question - that would mean to ignore Heisenberg.
But you may measure you had one - even with a specific wavelength.
See also
Frequency and wavelength of photons
and
Can a photon exhibit multiple frequencies?
discussing that a photon may be a superposition of multiple frequency states - before measuring, of course.
