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Colour Vision
The visible light spectrum is only a small section of the light spectrum.
The properties of light waves determine what we see:
Wavelength - determines hue (colour)
Amplitude - determines brightness
Purity - determines saturation.
Trichromatic Theory
According to trichromatic theory, we have 3 types of cones that are sensitive to red, green, and blue light. Combining nearby cones allows us to see the full range of colours.
This fits well with evidence from colorblindness - people tend to lose sensitivity to one of these colours.
Opponent Process Theory
According to opponent process theory, we have 3 types of cones.
Each type makes an antagonistic response to a pair of colours - a red/green cone can only be red or green at a particular moment.
This fits well with evidence from afterimages - when you stare at something red for a long time and then look at a white page, you see an afterimage in green.
It turns out that a combination of trichromatic and opponent process theories explains colour vision best.
We do have cones that are most sensitive to red, to green, and to blue (but really they respond to a range of colours).
Once the signal makes it past the retina, there are other cells that respond in an opponent process way.
Practice: Colour Vision
Which theory explains the way you see colour due to three colour receptors in the eye?