# Why do you need at-least three colors to make every other color?

by Avichal
Tags: color vision, colour vision
 P: 261 Since with the help of two different wavelength of light we can make every other wavelength of light why do we need three then - like red, green, blue or red, green, yellow? I guess red, blue or red, yellow will suffice.
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Hi Avichal!
 Quote by Avichal Since with the help of two different wavelength of light we can make every other wavelength of light …
nope

you can't make a single wavelength out of any other wavelength(s)
 … why do we need three then - like red, green, blue or red, green, yellow?
humans need three, because that's the way the human eye works

(i think some humans actually need four … so they sometimes see two things as different colours although most humans see them as the same colour)

each colour receptor responds to all wavelengths, but in unequal amounts

the response is a sort of bell curve, with a maximum in the red green or blue
see http://en.wikipedia.org/wiki/Color_vision for details
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Quote by tiny-tim
 Quote by Avichal Since with the help of two different wavelength of light we can make every other wavelength of light ...
nope

you can't make a single wavelength out of any other wavelength(s)
To elaborate, suppose you have a white wall, a red laser, a yellow laser, and an orange laser. Aim the red and yellow lasers at the same spot on the wall and aim the orange laser at a different spot on the wall. Next tune the intensities of the red and yellow lasers so that the colors of those two spots is indistinguishable to the human eye.

Just because those two spots are indistinguishable to the human eye does not mean that they have the same wavelength. That they appear to be the same color is just an optical illusion based on how the human eye works.

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## Why do you need at-least three colors to make every other color?

 Quote by D H ... Just because those two spots are indistinguishable to the human eye does not mean that they have the same wavelength. That they appear to be the same color is just an optical illusion based on how the human eye works.
And to add a little color to the thread

 ... there are three types of color-sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue sensitive detectors.
The Color-Sensitive Cones
 P: 191 ... and not forgetting that many colours don't have a "wavelength" at all as they don't exist on the spectrum.
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 Quote by NemoReally ... and not forgetting that many colours don't have a "wavelength" at all as they don't exist on the spectrum.
E.g., purple. Or white.
P: 1,060
 Quote by D H E.g., purple. Or white.
Thought it was pink..

But yeah, nothing to do with actual colors or light, just part of our biology. Our vision is trichromatic.
 P: 261 Oh ok....its not the light or color. Its how our eyes work! So there is a possibility that some animals need only two colors i.e. they are dichromatic(or whatever it is called)?
P: 284
 Quote by Avichal Oh ok....its not the light or color. Its how our eyes work! So there is a possibility that some animals need only two colors i.e. they are dichromatic(or whatever it is called)?
Yes.

Almost all mammals other than monkeys are dichromatic, like colourblind. Not monochromatic, though.
P: 261
 Quote by snorkack Yes. Almost all mammals other than monkeys are dichromatic, like colourblind. Not monochromatic, though.
Why is dichromatic colourblind?
P: 191
 Quote by Avichal Why is dichromatic colourblind?
In a normally trichromatic (or better) species, it means that the animal lacks the ability to see the full normal colour "spectrum". A dichromatic human is regarded as colour blind (even though they can see some colours) but I'm not sure that a normally dichromatic species can be properly be regarded as colourblind. Using that definition, then you could probably make a case for humans being normally colourblind as they lack the colour range of a tetra- or penta-chromat. Indeed, we have no idea what colour really is and, consequently, what possible values it could take. I think a monochromat could be regarded as colourblind as there is no indication that they see "colour" at all.
 P: 152 Why do we need four colours to make a map, if we can only see three?
P: 191
 Quote by Alfi Why do we need four colours to make a map, if we can only see three?
The 2 questions are not related. Stealing shamelessly from Wiki ..
The 4-colour map theorem states that no more than four colors are required to color the regions of the map so that no two adjacent regions have the same color, where two regions are called adjacent if they share a common boundary that is not a corner, and corners are the points shared by three or more regions. In fact, the word "colour" in this context is effectively just a label to indicate some unique marking (eg, letter, number or shading); for example, you could replace 'blue', 'green', 'red', 'yellow' with 'a', 'b', 'c', 'd', in which case you could call it the "4-letter problem"!

As for our vision, we can see significantly more than 3 colours - just open up the colour picking palette in Paint. As indicated in earlier threads, all things being equal, our eyes possess 3 specialized sensors (the cones) that have differing sensitivities to light of different wavelengths (energy, frequency, take your pick). Our brain then (somehow) combines the outputs from adjacent cones to construct "colour". A (speculative) reason we see so many colours is to aid identification of fruits, plants and other objects that appear visually similar in grayscale but are quite different in their edibility or danger.
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 Quote by 1MileCrash Thought it was pink..
Pink is a non-spectral color, but a rather boring one. The same goes with light green, light blue, light anything. Pink is just light red, basically a white spectrum but with a bit of a peak in the red. (BTW, white is also non-spectral.)

Purple and magenta are much more interesting non-spectral colors. Suppose you shine a green laser and red laser at the same spot on a white wall. You'll see a yellow spot, a spectral color whose frequency is intermediate between the frequencies of the green and red lasers. A similar thing happens with a blue laser and a green laser. Now you'll see cyan, another spectral color.

Use a blue laser and a red laser and you'll get something very different. You don't see green, which is intermediate between blue and red in terms of frequency. You see magenta. Or purple. Or some other non-spectral color that in our heads is very, very different from green. Purple and violet are visually similar colors, yet spectrally they are very different.

 Quote by NemoReally Our brain then (somehow) combines the outputs from adjacent cones to construct "colour".
And what it does to make the linear range from red to violet wrap around to instead form the color circle is part of that "somehow". Purple is a pigment of our imagination.
P: 191
 Quote by D H ... Purple is a pigment of our imagination.
Ouch! If you send me your address, I'll arrange delivery of a smart bomb ...
P: 261
 Quote by NemoReally In a normally trichromatic (or better) species, it means that the animal lacks the ability to see the full normal colour "spectrum". A dichromatic human is regarded as colour blind (even though they can see some colours) but I'm not sure that a normally dichromatic species can be properly be regarded as colourblind. Using that definition, then you could probably make a case for humans being normally colourblind as they lack the colour range of a tetra- or penta-chromat. Indeed, we have no idea what colour really is and, consequently, what possible values it could take. I think a monochromat could be regarded as colourblind as there is no indication that they see "colour" at all.
So can you say that a trichromatic species sees more colors than dichromatic and similarly higher i.e. tetra or penta see even more.

Now I feel as if I don't know what exactly color is?
P: 191
 Quote by Avichal So can you say that a trichromatic species sees more colors than dichromatic and similarly higher i.e. tetra or penta see even more.
In principle, simply on the basis that there is an extra 'dimension' for colour to extend into (by analogy, consider the rooms available in a single story dwelling to those in a high-rise tower of the same footprint and rooms per floor). However, the colour discrimination might be lower in a particular trichromat than in a dichromat (back to the analogy, d = 30 by 30 rooms and t = 5 x 5 x 5 rooms) as a result of cone sensitivity and / or brain processing. In addition, the colour dimensions might not contribute equally (eg, a 5 x 20 x 10 block or even (5 x 20) and (10) or some rooms might cover more than one floor).

... whether any of these happens in reality, I don't know.

 Now I feel as if I don't know what exactly color is?
I don't think anybody knows what colour actually is. However, we can develop mathematical models that allow us to functionally describe and model it. You might the following webpage of interest ... http://www.archimedes-lab.org/color_...llusions.html#
P: 284
 Quote by D H And what it does to make the linear range from red to violet wrap around to instead form the color circle is part of that "somehow". Purple is a pigment of our imagination.
We actually have an idea how. Another picture claiming to be the graph of human visual pigment sensitivity. But with vital features missing above:
http://www.huevaluechroma.com/032.php
Note how the red pigment has a rise towards secondary maximum, from 450 nm shortward. While the blue falls from maximum at 420 nm - which the other picture claims 445 nm.

True violet, at 390-400 nm, causes less excitation of the blue sensors than true blue in the range of 420-450 nm, because true violet is blueward of the blue maximum sensitivity. At the same time, the true violet causes more actual excitation of red sensors than true blue (because true violet is already into secondary maximum) and especially more excitation relative to excitation of blue sensors (because the blues are less excited, as stated above). Therefore, monochromatic short radiation looks much like mixture of blue and red.

And for comparison, a graph claimed applicable to birds (specifically finches):
http://en.wikipedia.org/wiki/File:Bi...ensitivity.svg

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