What are all the ways in which a physical object or substance can appear to have colour, and how do they actually work?

Factual Questions
1

There’s an ongoing discussion/debate of which I seem to be on the periphery, about whether or not the colour blue exists in nature. It began with me making a statement that there are no blue sunflowers, to which there were various enthusiastically agreeing statements to the effect:

  • Yes, in fact there are no blue flowers at all!
  • Yes, in fact, blue doesn’t exist at all in nature!!!

I don’t believe either of these statements is correct; there are blue flowers such as Himalayan poppies, Ceanothus, Forget-me-not, Green Alkanet (below) and several other members of the borage family and others.

Denial of the blue colour of blue flowers seems to be largely a goalpost-moving no-true-scotsman exercise in which, rather than flowers not being blue, blue is carefully redefined to an absurdly strict degree that we would never apply to the consideration of any other colour of flower, so as to exclude the example being discussed.

The ‘no blue at all in nature’ thing appears to be a nuance-failure interpretation of the fact that things like the Blue Morpho butterfly use structural colour rather than pigment, to reflect blue light from their wings, but anyway, that got me to wondering, what are all the different ways in which ‘colour’ is produced.
I mean, I know the phenomenon we call colour is the result of specific wavelengths, or mixtures of wavelengths being detected by assorted receptors in our eyes and interpreted by our brains, but I’m not talking about the qualia, I mean how do natural things ‘be coloured’.

The probably-incomplete list in my head includes:
Selective absorption and reflection - an object is illuminated by white light, from which it absorbs some wavelengths and reflects others - the composition of the reflected wavelengths determines the colour (I think we can ignore things like the difference between spectral and composite perceived colours, unless it has specific relevance somewhere).

Selective absorption and reflection - as above, but wavelengths not absorbed are transmitted through a transparent or translucent material and the composition of these wavelengths determines the colour.

Thin film interference - is this fundamentally different from structural colour on a butterfly wing?

Emission - I suppose we have to include colour that arises because an object emits a specific band or blend of wavelengths - how many different ways are there for that to happen?

Scattering - the sky is blue because blue light is scattered more than other wavelengths as it crosses the atmosphere

Refraction can differentiate wavelengths from a mixed source and we see those differentiated wavelengths as colours

Also on the absorption/transmission thing, I suppose I need to ask are there multiple different mechanisms for that? Do all dyes, pigments and coloured chemicals work in the same general way as each other?

And what else is there that I haven’t mentioned? (fluorescence? Or is that included as just an emissive phenomenon?) And please correct me if I am wrong about any of the above - and please elaborate on any of the phenomena and their underlying mechanisms!

2

When light hits an object, three things happen, to various degrees: The light can get absorbed, reflected, or transmitted (and we can further divide up “absorbed” into subcategories). When we see something, we’re seeing light that’s absorbed by it, reflected by it, or emitted by it. I mention this because it complicates the question of what color something is. We can say, for instance, that the sky is blue because air is blue. But by that same token, we would also be forced to say that sunsets are red because air is red. For most physical, tangible objects that we’re familiar with, their color is due almost entirely to selective absorption of light, so that the color you see transmitted through them is the same as the color you see reflected off of them. Copper sulfate, for instance, is blue no matter how you look at it, via reflected or transmitted light. But air is blue when you look at it via reflected light, but red when you look at it via transmitted light.

3

Thank you - that’s helpful in narrowing the scope here - I think then the question is: what are the different absorption modes? - are there different ways in which light is absorbed?

4

Rose are red.
Violets are blue.
Be as pedantic as you want;
This rhyme is true.

5

A couple more:
Diffraction. Works much like refraction in that it’s frequency-dependent and will split a mixed source into components.

Frequency conversion. Comes in many varieties. White LEDs use a phosphor to convert blue light to yellow. Frequency-doubling crystals convert infrared light into green (for green lasers), among other combinations.

Phosphors would be a type of emission but I’d consider them distinct from direct emission, say from the LED chip itself (which is emitting photons of a certain frequency based on the electron bandgap).

Ultimately, though, all of these result in photons that enter your eye. And the only distinguishing characteristic there is how differing mixes of colors might produce the same perceived color, which is called metamerism. That is, if you perceive yellow, it could have been a pure yellow frequency (about 570 nm) or a mix of red+green, or an infinite number of other combinations that stimulate your color receptors in the same way. Beyond that, all photons of the same frequency are identical, and it doesn’t matter at all how they were produced.

6

Sure, but it’s the how they are produced that is the question here.

How - that is, by what mechanism - does an organic pigment such as Cyanidin absorb/reflect light in such as to appear purple? Is it the same mechanism that results in different chemicals (such as copper sulphate, potassium permanganate etc) appearing the colours they appear?

7

The answer to that is always “it’s the electrons”. But beyond that there’s basically fractal depth to how complicated the answer can be. For cyanidin, if Google is to be believed, the answer has something to do with delocalized π-electrons from its aromatic, cyclic ring structure. On the other hand, why is gold gold? That has to do with relativistic electrons in the 5d and 6s orbitals. What about diffraction gratings? Well, those are simpler, requiring only a regular array of reflective strips… which are reflective because metals have free electrons in their conduction band which absorb and re-emit light.

And so on. There just isn’t going to be an all-encompassing answer except that it’s about the electrons. Color is produced in lots of interesting ways, but the answer to your debate partners is that if it looks blue, then it’s colored blue, regardless of the mechanism. The only tiny quibble has to do with metamerism, where instead of color X you might have a mix of other colors that appear the same to your eye.

8

If you really want a solid grounding in the physics of light, I’d recommend the Feynman Lectures on Physics, Vol 1, starting at Chapter 21ish. It is not an easy read. And it has very few specific answers to these kinds of questions, with the exception of why the sky is blue, which it does work out in full detail. But it will give you a broad mental framework for thinking about questions of this nature. Everything is then just a special case of that understanding, which you can pursue in more detail if desired.

9

Yeah, although with blue being at one end of the visual spectrum, that’s less likely to be happening - there’s not really any two non-blue wavelengths of light you can mix, such that the result will stimulate the human perceptual system to see blue.

10

The mechanism by which something has color can be relevant, though. If you take rose petals and grind them into a powder, you’ll have a red powder. But if you take blue bird feathers and grind them into a powder, you’ll have a grayish-white powder (and I think the same is true of most blue flowers). It means something different to say that roses are red versus saying that jay feathers are blue.

11

The definitive book on this is Kurt Nassau’s The Physics and Chemistry of Color: The Fifteen Causes of Color

12

Different branch of science, but:

The OP reminds me of how grade-schoolers are taught about several types of “simple machines” (e.g. inclined plane, pulley, lever, wheel-&-axle, wedge, etc.) … and then years later on – say in college or an advanced high-school course – they learn that all these “simple machines” are variations on the same single “machine” (all “levers”, sorta kinda sorta).

The various types of color production seem to be like the classification of simple machines. You can lump a bunch of color-production schemes together into “kind of the same process with trivial differences”. Or you can split between the fine points of difference and come up with dozens of kinds of color production.

13

Maybe, but I don’t think that indicates that blue flowers have anything in common with blue bird feathers - the blue flowers in the opening post contain specialised vacuoles that contain the cyanidin-3-glucoside anthocyanin - which is normally a purple colour but it is also a pH indicator ranging from red in the presence of acid to blue with bases - the plant maintains the interior of these vacuoles in an alkaline state (which is quite a feat for plants), causing the anthocyanin to turn blue. Grinding up the flowers introduces oxygen and in other ways destroys the setup and the extracted anthocyanin reverts to a more purple colour.

14

Ah, right, I’d forgotten about indicator pigments. I think that at least some blue plant parts are structural colors like feathers, but you’re right that that’s not the mechanism behind at least some of them.

And I didn’t know that some plant colors were via indicator pigments that are maintained at a particular pH inside the plant. Fascinating.

15

Hydrangeas are even more weird - the flowers turn blue when the soil is acidic and red/pink when it’s alkaline - this is the opposite of expected for the typical pigments. Something to do with the availability of aluminium ions I believe.

16

Those should be named sumtil plants, not hydrangea.

17

I think some plants use other methods to appear blue - sea holly (eryngium) and hedgehog thistle (echinops) have a sort of metallic blue lustre - might be structural colour of some sort

18

The one color none of us will ever see is olo. Google Search

It is said to be a very intense green, but very special technology is required to see it.

19

That’s a neat trick :slight_smile:
I mean, I think I know what you meant, but isn’t the “not absorbed by it” covered by reflection?

20

I suppose you could say that we see an object coated in VANTA black because it absorbs (nearly) all visible light - we see a shape that we assign the colour ‘black’, even though what we are perceiving is the absence of light, in contrast to and juxtaposed against all the other objects in scene, which are reflecting something.