exchemist: Yes. The red and green cones have quite a large degree of overlap in their frequency sensitivities, which is one reason why red-green colour blindness is the most common kind. The blue cones peak a bit further away frequency-wise (or wavelength-wise). Yellow light does have an actual frequency, somewhere between green and red light. But, as you said, our perception of every frequency depends on how strongly each type of colour cone responds to the incoming light. That means we can, for example, create a perception of yellow by mixing appropriate amounts of green and red light, for instance, rather than using actual yellow light. In fact, this is why computer displays (LEDs, even old-fashioned phosphor screens) only need three colours for each pixel (RGB). By varying the intensities of the three channels, we can produce a sensory experience indistinguishable (to us) from using a frequency of light somewhere between the peak values of the RGB emitters. It's interesting to think about how a creature with more sophisticated colour vision would perceive our RGB screens. They would not think that they reproduce all colours accurately, for instance.