Principle of univariance
The principle of univariance is how one can discriminate between wavelengths through comparison of multiple photoreceptors.
[1] The principle states that one and the same visual receptor cell can be excited by different combinations of wavelength and intensity, so that the brain cannot know the color of a certain point of the retinal image.
Thus the wavelength information can be extracted only by comparing the responses across different types of receptors.
However, the brain's cells can only vary in the rate at which action potentials are fired.
To avoid this, the response of multiple cells is compared.
In column 1, a ball is illuminated by monochromatic light. Multiplying the spectrum by the cones' spectral sensitivity curves gives the response for each cone type.
In column 2, metamerism is used to simulate the scene with blue, green and red LEDs, giving a similar response.