Why are the chelicerae of  Phidippus Audax  iridescent green?
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Iridescence is quite common in insects and birds (and yes, spiders).  \n\nIridescence is when light reflects from *very* thin layers of semi-transparent material (think of the rainbows you see on the surface of a CD or soap bubble).  Some light reflects from the outer surface, and some penetrates a short distance before being reflected off an inner surface. The two types of surface are at a specific angle.  The wavelengths of the two types of reflection interfere with each other, creating new colors at specific angles.  (Incidentally, the word comes from the Greek 'iris', meaning 'rainbow'.)  That is iridescence.\n\nIn insects and spiders, the iridescence is caused by layers of something called 'chitin', the horny material that makes up the external skeleton of insects.  The layers of chitin are about a thousand times thinner than a human hair.
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Emerald green is a sexy eye color. It makes the eyes sparkle.
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They are bright because they have pigment molecules in their petals that absorb particular wavelenghts of visible light, making them appear colored.\n\nBut the reason they evolved those pigments was to attract pollinators.
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http://www.infoplease.com/ipd/A0587677.html\n\nhttp://www.biology-online.org/dictionary/phytochrome
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Green. Carotene sends the light it absorbs on to it's old friend, chlorophyll. Ever see the leaves of carrots?
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There are actually 2 types of chlorophyll, named a and b. They differ only slightly, in the composition of a sidechain (in a it is -CH3, in b it is CHO). Both of these two chlorophylls are very effective photoreceptors because they contain a network of alternating single and double bonds, and the orbitals can delocalise stabilising the structure. Such delocalised polyenes have very strong absorption bands in the visible regions of the spectrum, allowing the plant to absorb the energy from sunlight.\n\nThe different sidegroups in the 2 chlorophylls 'tune' the absorption spectrum to slightly different wavelengths, so that light that is not significantly absorbed by chlorophyll a, at, say, 460nm, will instead be captured by chlorophyll b, which absorbs strongly at that wavelength. Thus these two kinds of chlorophyll complement each other in absorbing sunlight. Plants can obtain all their energy requirements from the blue and red parts of the spectrum, however, there is still a large spectral region, between 500-600nm, where very little light is absorbed. This light is in the green region of the spectrum, and since it is reflected, this is the reason plants appear green. Chlorophyll absorbs so strongly that it can mask other less intense colours. Some of these more delicate colours (from molecules such as carotene and quercetin) are revealed when the chlorophyll molecule decays in the Autumn, and the woodlands turn red, orange, and golden brown. Chlorophyll can also be damaged when vegetation is cooked, since the central Mg atom is replaced by hydrogen ions. This affects the energy levels within the molecule, causing its absorbance spectrum to alter. Thus cooked leaves change colour - often becoming a paler, insipid yellowy green.
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scent and color. Insects see color.
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Because leves contain pigment Chlorophyll, which absorb all colors except green.That is why!
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Color variations would be for camouflaging itself against it's prey.  Emerald green eyes are usual for deep sea bottom dwellers, such as the chain dogfish. Crescent shaped tails are for speed and agility in the water.
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>>>why most common chlorophyl on most leaves is the the\n>>> green one? not the red one, or other color instead...\n\nChlorophyll only comes in one colour: green. There are red and yellow accessory pigments, but they are not chlorophyll and serve only to transfer energy to chlorophyll. The chlorophyll is still required to use those accessory pigments.\n\nAs for why chlorophyll predominates, it is simply because chlorophyll blue light, and red wavelengths are the most common wavelength in sunlight. Or to put it another way, chlorophyll has evolved to absorb that part of the sunlight where the most energy is.\n\nIf leaves were any colour but green they would be wasting most of the energy in sunlight.
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The colouration may not make as much sense on land, when they are waddling about, but in the sea as they 'fly' through the water in search of fish and squid and other yummies, it is actually a highly effective form of camouflage.\n\nCalled counter-shading, it makes them pale from below, blending into the brighter background of the surface, and dark from above, blending into the stygian blackness of the abyssal depths.\n\nA lesser form of counter-shading is common in many terrestrial critters as well, where belly fur is often paler than the pelage on the dorsal surface.  Dogs, kitties, even squirrels and bunnies often show this same pattern.\n\nIn the watery environs of a pelagic predator, it is even more effective due to the greater degree of contrast in the environment, and the lack of effective cover in which to hide.  \n\nCounter-shading can be seen in critters like sharks, dolphins, swordfish,  and tuna.\n\nHere's a picture of a tuna showing counter-shading http://www.panamafishingandcatching.com/tuna/preultimo-tuna-tunalarge.jpg\n\nAnd one of a shark http://na.nefsc.noaa.gov/sharks/img/27.jpg\n\nProbably the best examples of counter-shading would be the orca (aka killer whale), and the manta ray.  Like penguins, they are very distinctly black on the back and white underneath.\n\nManta ray http://www.no-pest.com/MantaRay.jpg\n\nOrca http://orca-voyance.auraciel.com/orque2a.jpg
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As to where the color itself comes from:\n\n"The wings of butterflies and moths consist of a colorless translucent membrane covered by a layer of scales (the name of the order is Lepidoptera, meaning 'scaly wings'). Each scale is a flattened outgrowth of a single cell and is about 100 µm long and 50 µm wide. The scales overlap like roof tiles and completely cover the membrane, appearing as dust to the naked eye. \n\nIn butterflies, iridescence is caused by multiple slit interference. Sunlight contains a full range of light wavelengths. "Interference" occurs when light hitting the wing combines with light reflected off the wing. Light is a wave. If the crests and the troughs of the waves are aligned, or "in phase", they will cause constructive interference and iridescence. This happens when the one light wave hits the first groove, and a second light wave travels half of a wavelength to an other groove and is reflected back in phase with the first. If the crest of one wave meets the trough of an other wave ("out of phase"), they will cancel each other out, demising the overall light intensity."\n\nAs to WHY they are colored, this answer applies to Morpho Blues, but can be applied to other butterflies as well:\n\n"Iridescent surfaces, such as butterfly wings, help animals to elude potential predators. When these insects fly, the upper surface of their wings continually changes from bright blue to dull brown because the angle of the light striking the wing changes. As the butterflies move their wings up and down during flight, they seem to disappear and they reappear a short distance away, looking like ethereal flashes of bright blue light. The dark undersides of their wings strengthen this effect. Combined with an undulating pattern of flight, this ability to quickly change color makes them difficult items for predators to pursue."
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The sunlight gives cllorphyl a pigment to turn green when thiers less sun the other pigments reflect because the subnlight is not as strong as b4
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There's a prominent red emission line of hydrogen that dominates the color profile of emission nebulae for CCDs and most films. The eye, however, is not very sensitive at that wavelength, so it's the blue-green hydrogen emission that you see. Astrophotographs are generally color-balanced to show the contributions of significant spectral lines, rather than to reproduce any sort of "realistic" color.
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Reduction in light and in some cases lower temps for some trees kill the chlorophyll.  This lets the other pigments(reds and yellows) show through that have always been there but were hidden by the abundant chlorophyll.
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it is because of pigment (probably xanthin) which emmits wavelength of yellow light and absorbs all others
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Frogs come in variety of colors but the majority of them are green. Being green when sitting on green leaves has obvious advantages if you want to avoid becoming an easy meal for some hungry predator. \n\nFrogs are not green because they have green pigment in their skin.In their skins they have three types of pigment cells (called chromatophores) stacked on top of each other. At the bottom are melanophores, containing a mostly dark pigment called melanin. These are the same cells that can make human skin various shades of brown. On top of the melanophores are iridophores, packed with highly reflective bundles of purine crystals, and on top of the iridophores are xanthophores, usually packed with yellowish pteridine pigments. In the typical green frog, light penetrates to the iridophores, which act like tiny mirrors to reflect mostly blue light back into the xanthophores above them. These cells act like yellow filters, so the light escaping the skin surface appears green to our eyes.\n\nBy manipulating all three types of pigment cells, a wide range of colors can be produced, although usually the range extends from bright green to various shades of brown and gray. In frogs, all of these changes appear to be mediated by hormones circulating in the blood. The advantage of such color change is obvious. Imagine a frog leaping from a green leaf onto a brown tree branch. Melanin moves, reflective purine crystals shift position, yellow pteridine pigments cluster or disperse, and voila, that green frog that stood out like, well, like a green frog sitting on a brown branch is now a well camouflaged brown frog.