Spectral discrimination occurs when more than one photoreceptor is present. For example, mammals are dichromats, they being able to distinguish hue differences in violet to green range of the spectrum, while birds are tetrachromats, being able to distinguish hue differences in very large spectral ranges, including ultraviolet light.
Humans are trichromats, meaning that they detect colors in green, blue and red ranges, through photoreceptors called cone cells.
Achromatopsia represents a rare genetic disorder that occurs when cones become defective and it causes the inability to distinguish the colors.
Deuteranomaly is a genetic inherited condition, due to opsin pigments, that is present in 6 to 8% of humans and it causes the inability to distinguish the colors in red and green ranges but it increases the ability to distinguish hues of khaki color.