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After discovering flamingos natural colour isn't red, pinkish as one naively may have assumed though, I looked into the matter more deeply. I also found out that many other animals do not produce the respective colour pigments matching then their outer appearance.

Some seem to use certain optical effects like Rayleigh Scattering or applying several different layers of filters to absorb wave lengths of light from the spectrum just emitting the intended colour.

After browsing through some more cases several questions arose, which of course cannot be answer all in detail here. However most interesting to me were:

  1. Why does it seem so difficult for animals in contrast to plants to produce their own colour pigments?

  2. Some colours seem to be more difficult to achieve than others. For example I found that melanin and several variation of it as colour pigments are produced quite commonly by animals, however blue rather difficult. As the following post indicates, blue colour pigments seem to be in general difficult to produce: Why are so few foods blue?

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    $\begingroup$ As my previous comment on the question suggesting improvements has been read I have removed it. I'd suggest you clear your own when you have made modifications. That way just the science will be left, and, as I have said, I think it is worth discussing. If you don‘t get round to editing it yourself, I‘ll do it. Editing of questions and answers is encouraged where it will improve them. $\endgroup$ – David Feb 21 '17 at 22:49
  • $\begingroup$ @David, I edited parts but you are always welcome to edit / expand the question further. I consider myself not very well versed, maybe naive in the topic and thus miss certain opportunities. Better there are more questions / thoughts put together encouraging thinking and constructive discussions than ideas missed and lost. $\endgroup$ – Imago Feb 22 '17 at 11:47
  • $\begingroup$ Fine. If I ever get time I'll look in more detail into the metabolic pathways involved (although I'm not sure that this is your field.) As far as the patch of blue is concerned, I imagine that we are getting into colour chemistry as well as metabolism. The story of indigo comes to mind. However the title of the other post puts me off. Is it concerned with edibility or abundance? But I have given that question a wide berth. $\endgroup$ – David Feb 22 '17 at 19:32
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Although no expert in this field, I think this question is interesting and I would like to put forward some tentative answers.

In a general evolutionary sense one might regard coloration as a secondary feature of living things — especially animals. Their primary concern is to obtain energy, synthesize structural components, reproduce etc. So the metabolism that evolves under this pressure is such as to allow them to do these things. Colour is not a primary concern. Pigments are complex molecules, and enzymes are only likely to evolve to produce them if there are already reasonably complex precursors available. I know little about plant biochemistry, and would welcome improvement of this answer, but it would seem to me that (1) plants have more complex or different metabolic pathways than animals, such as those for synthesizing chlorophyl, and (2) the pathways for producing pigment precursors are more widespread, perhaps because coloration affords a greater advantage to plants — for pollination etc.

There does appear to be pigment synthesis in animals — an article on pigmentation in bird feathers can be found on the website of The Cornell Lab, for example. That states that the pigments in bird feathers are based on three types of compounds: carotenoids (red or yellow), melanins (black to reddish brown and pale yellow) and porphyrins (pinks, browns, reds and greens).

Two of these are related to compounds the birds synthesize themselves from amino acids — melanins for skin pigmentation, and porphyrins for haem. However, it appears that carotenoids are not synthesized by the birds that employ them for coloured feathers, but are acquired from plants in their diet.

Finally, one might argue that producing colours by optical effects has certain advantages over the use of pigments, namely that they are ‘tunable’ — can be modulated or turned on and off.

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    $\begingroup$ I like this answer, but it seems to me that colouration need not always be a secondary concern. Colouration has evolved, for example, as camouflage or for mimicry. $\endgroup$ – canadianer Feb 20 '17 at 23:16
  • $\begingroup$ I agree that it is important for those animals that use it. And of course they don't care how they achieve it — pigmentation or optics. My thought was just that central metabolism came first — indeed the terminology "secondary metabolism" reflects this. The pathways leading to anthrocyanin production seem to be widespread in plants, and quick googling indicates that anthrocyanins are made via the phenylpropanoid pathway, which apparently is used for other important non-pigment plant functions. Hence it would seem a more general plant feature. (Where are the plant biochemists when you…?) $\endgroup$ – David Feb 20 '17 at 23:41
  • $\begingroup$ Ah, you're thought is well taken. $\endgroup$ – canadianer Feb 20 '17 at 23:46

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