The threshold for cone activation is much higher than that for rods because cones only respond to high intensity light.

The visual pigments of cones, like rods are a combination of retinal and opsins.  The opsins of cones are referred to as photopsins.  Unlike rods, there are three distinct types of cones, each sensitive to a different wavelength range, reflecting the specific properties of photopsin to which the retinal is bound.  The three types of photopsin are different to each other and scotopsin.  The different cone opsins provide a unique electrical environment for its retinal.  This determines the retinals sensitivity to light.  The naming of the cones represents the colour that each type of cone absorbs.

• Blue cones: Respond maximally to a wavelength of 455nm.

• Green cones: Respond maximally to a wavelength of 530nm.

• Red cones: Respond maximally to a wavelength of 625nm.

Image taken from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html without permission

However as shown there is overlap in the absorption spectrum, and perception of intermediate colours results from a simultaneous activation from more than one type of cone receptor.  Yellow light stimulates both red and green cone receptors.  If the green receptor is stimulated more than the red receptor a orange colour will be perceived instead of yellow.  When the cones are all simulated equally we see white.  Our perception of brightness and saturation is due to the number of cones stimulated.

Colour Blindness

Colour blindness is due to the lack of one or more type of cone receptor.  It is an inherited condition and is more common in males.  The most common type of colour blindness is red-green.  This is due to a deficiency in either red or green cones.  The condition results in red and green being seen as the same colour (either red or green), depending on which cone cells are missing.