I work a lot with infra-red lasers, and these are not visible to the naked eye, nor would an ultraviolet laser be visible in a dark room. Where high spectral purity is important (eg long-distance telecommunications), you can couple a second cavity so that the selected wavelength is enhanced, and the unwanted wavelengths suppressed. But these wavelengths are still so close together that the human eye cannot distinguish them.
In practice, lasers do not produce exactly one wavelength - they typically produce several closely-spaced wavelengths, which are a submultiple of the length of the laser cavity. So a laser can produce pure "rainbow" colors, but not "mixed" colors like brown, cyan or magenta. But you cannot have a beam which somehow negates the presence of other light sources in the way you mean.Ī laser beam is defined as being a single wavelength coherent light. Well I suppose you could switch the light source off and, if there were no other light sources, you would have black. As Chemistry4me implied, you can't have black as black is the absence of light. What we see as a spectrum of colours is made up of the colours R, G and B. This is because the receptors' bands overlap quite a lot so that light at a wavelength between Red and Green stimulates Red and Green receptors and the brain interprets this in the same way as if it were a single wavelength between Red and Green.
A TV only uses Red, Green and Blue but gives the impression that you can see Orange, Yellow and Violet too. What we observe as a colour is because the human eye has receptors that respond to different wavelengths. I would suppose you could have different coloured lasers (say R, G and B) accurately aligned to each other to give the impression of a single beam that could produce any colour by changing the relative intensity of the lasers. It is a prime feature of all lasers that they can only be designed for one wavelength (or colour). These lasers can also produce a single colour and each laser will only work in a very narrow wavelength band. The lasers you are referring to are high powered (pumped) lasers and the wavelength of light produced depends on their construction and the materials used. Other colours can be created by using two or more LEDs of different wavelengths or by using a phosphor to absorb the monochromatic light and re-emit it over a different wavelength band (like in a fluorescent tube). LEDs are similar but the construction is simpler and the spectrum is wider, but is still quite narrow and just one colour. Specifically designed laser diodes produce light at one wavelength only that is one colour only.