Pentachromacy describes the capability and capacity for capturing, transmitting, processing, and perceiving five independent channels of color information through the primary visual system. Organisms with pentachromacy are termed pentachromats. For these organisms, it would take at least five differing ranges of wavelengths along the electromagnetic spectrum to reproduce their full visual spectrum. In comparison, a combination of red, green, and blue wavelengths of light are all that is necessary to simulate most of the common human trichromat visual spectrum.
One proposed explanation for pentachromacy is a retina containing five distinct types of cone cells with differing absorption spectra. In actuality the number of cone cell types may be greater than five as different types may be active at a specific intensity or range of intensities for a given wavelength of electromagnetic radiation.
Animals that are potentially pentachromats
Some birds (notably pigeons) and butterflies have five or more kinds of color receptors in their retinae, and are therefore believed to be pentachromats, though psychophysical evidence of functional pentachromacy is lacking. Research also indicates that some lampreys, members of the Petromyzontiformes, may be pentachromats. It is suspected that a human female could inherit multiple alleles for color blindness as protanomaly, deuteranomaly, and/or tritanomaly leading to the phenotypic expression of at least four and possibly as many as six types of color-sensing cones, although the red-, green-, and blue-deficient cones would have degenerate spectral sensitivity. 
Color range in pentachromats
According to Jay Neitz, a visual spectrum researcher at the University of Washington, the three common types of cones in the human retina — long, medium, and short — can each distinguish approximately 100 intensities of light throughout their continuum of sensitivity within the visual spectrum. The visual cortex in the occipital lobe of the cerebrum can multiplex these varying intensities which allows a typical human to distinguish approximately one million discrete hues. Theoretically, a pentachromat, assuming the same spectral resolution of 100 intensities for each of five cone cell types and the same cognitive combinatorial capacity, may be capable of distinguishing up to 10 billion colors.
Research shows that animals sensitive to more than three color channels are likely to see the world in a very different way from humans. These animals are likely to experience different and more numerous unique hues, along with additional ways of mixing them.
- Wavelength discrimination in the ‘visible’ and ultraviolet spectrum by pigeons, Jacky Emmerton1, and Juan D. Delhis, Journal of Comparative Physiology A. Volume 141, Number 1. March, 1980
- Functional characterization, tuning, and regulation of visual pigment gene expression in an anadromous lamprey -- Davies et al., 10.1096/fj.06-8057com -- The FASEB Journal
- Mark Roth (September 13, 2006). "Some women who are tetrachromats may see 100,000,000 colors, thanks to their genes". Pittsburgh Post-Gazette.
- "Color Vision:Almost Reason for Having Eyes" by Jay Neitz, Joseph Carroll, and Maureen Neitz Optics & Photonics News January 2001 1047-6938/01/01/0026/8- Optical Society of America