Well, it's pretty clear that if my monitor has three leds, each one emitting light at different frequencies, the only thing that they can produce is the sum of lights at those frequencies, not another frequency.

And viceversa: there are colors that come from mixtures of frequencies and you cannot have with one frequency (purple).

Having said that, we only have three types of receptors, and it is possible to match the response of those as good as possible.