i kinda understand the difference between subtractive and addictive colours but i don’t understand why we use green instead of yellow for lights
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When you observe light that shines directly to you (from the sun, from a light bulb), you observe additive colors. The photons go directly into your eyes, and their frequencies “add up” in such a way that the red plus GREEN plus blue frequency equals white.
On the other hand when you are looking at diffused light, for example from a printed paper, the photons from the sun first interact with the paper, and are either absorbed into the paper or reflected to you. If you’re seeing yellow, that means the blue photons have been absorbed, leaving the red and the green to enter your eyes.
The printer inks therefore work with subtractive colors (the ABSENCE of blue, the ABSENCE of green, the ABSENCE of red) in order to print an image in such a way that not printing anything (nothing absent) results in white. The ink colors are cyan, magenta, and yellow, which basically “subtract” red, green, and blue from the photons that will hit your eyes.
It’s basically designed this way so that:
* For direct light, “all” the colors results in white. Computer screens shine / create the light, so powering up all the colors in a pixel will display a white splotch. The typical movie or game screen is rather dark, very rarely is it white with a few words on it.
* For printed paper, “no” color makes you see the white paper, whereas using all colors makes you see black. Powering up all the ink sprayers will create a black splotch.
We usually print a few words or an image on white paper. The majority of the space on the paper is left blank (white). Having to actually “print” / ink a black paper to white would use up a lot of colors. Whereas printing black things on white paper saves color.
Red Green and blue are the primary colours for additive and yellow magenta cyan are the subtractive colormixing.
Yellow and blue only make green in subtractive mixing.
Because red, yellow and blue are the primary colors in a subtractive color space. Light, on the other hand, works with additive colors. In an additive color space, the primary colors are red, green and blue.
There aren’t actually any specific “primary colors”. You can use many different trios or color as “primary colors”. RGB is the one that most directly represents what we see, since it’s the trio of colors that our eyes react to, but you could use yellow instead of green or Cyan Magenta and Yellow or other color systems.
As long as they combine right, it works.
Firstly, there is no such thing as true, correct primary colours. They all are arbitrary choices, and they all have faults. It’s only a question of how good is it. Red, yellow, blue is a valid additive and subtractive primary colour scheme. It’s just not very good.
Humans have three types of cone cells in our eyes. Cone cells are what pick up coloured light and let us see with colour vision. This means if you are picking a set of primary colours, the best choice is three. One or two will be very limited to a human, and four or more will give a wider gamut (range of possible colours a human cam see), but is overkill with diminishing returns. A TV for dogs would probably only need two, and a TV for birds would probably need four. Different animals have a different amount of cone cell types.
So what do the three cones cells in the human eye pickup? Well, we call them the Long, Medium, and Short cones. Referring the the wavelength. Long is good for reds. Short good for blues. And mid is good for greens. But it’s not quite that simple, they respond to a range of wavelengths. And M and L overlap a LOT. [Here is a graph](https://en.m.wikipedia.org/wiki/Cone_cell#/media/File%3ACone-fundamentals-with-srgb-spectrum.svg).
Now, taking a look at that graph, it’s pretty clear why yellow is a poor choice for a primary colour. It’s closer to the L than the M. That means if we used yellow and red as primary colours, they both would cause the L cells to respond more than any other cone type. Definitely not ideal.
Is red, green, blue the perfect choice? No. In fact, if you did four primary colours with yellow included, you’d get a wider gamut.
You can represent this in a [graph called a colour space.](http://www.deprintedbox.com/blog/wp-content/uploads/2014/03/color-space-diagram.jpg). Don’t worry really about how you get this graph or what X and Y means, but this show all the colours a human can see. Don’t take the colours too literally, your phone or monitor can’t display all the ones you can actually see. The outside curve of the graph is the rainbow colours, the numbers are the wavelength. You can see the standard sRGB primary colours form a triangle in this space. Anything inside, a RGB display can show you. Anything outside, it cant. This triangle isn’t the whole space, sRGB is not perfect. There are green hues a TV or phone can’t show you.
Also shown is a hexachroma, or six primary colours space. Also shows a better Adobe RGB triangle, using more pure tones of RGB closer to the rainbow, to get a larger triangle. That’s also why QLED, or quantum dot LED, TVs are all the rage. The quantum dots are just fancy ways to make the pixels a more pure tone of each colour, getting a wider triangle.
Also shows CMYK, or cyan, magenta, yellow, black, what printers use. A subtractive colours space. Red, yellow, blue, the subtractive (paint, pigment) primary colour scheme many are taught in grade school is also a very poor choice. CMY is way better, but most kindergarteners don’t know what magenta is, so use red. And blue for cyan.
Now imagine you replaced green with yellow on the sRGB triangle. See how much the triangle would shrink in area? That’s why we don’t use red, yellow, blue. It would suck.
You can make any primary colour scheme you want. Just pick colours on this graph and draw a polygon with them. The more pure the colours are, and the more you pick, the larger you can make the area, and the better it is. The perfect primary colour scheme would to have thousand of primary colours all around the rainbow. Would perfectly capture human vision, but your TV would be a $2 million and need a server farm to store a single movie. It wouldn’t be worth it.
Because the red-blue-yellow primaries you learnt in primary school are wrong. Just one of the many, many, many[ lies to children](https://en.wikipedia.org/wiki/Lie-to-children) you were told then.
What do you mean?
Red, green and blue **are** primary colors.
Red, green and blue ARE the primary colors, we have receptors in our eyes that react to the wavelengths we call RED, GREEN and BLUE, the thing is that those receptors don’t only react to a single wavelength but many in different degrees so when you get a wavelength between green and red it activates both at the same time and your brain interprets it as yellow. In the same manner you can use a green light combined with a red one and since you’ll be activating both green and red receptors your brain will interpret it as yellow, this is called ADDITIVE color because you are adding light.
In graphic design we call CYAN, MAGENTA and YELLOW primaries because for printing we use SUBSTRACTIVE color, you start with white which reflects all the wavelengths and add pigments that absorbe some of them while reflecting only a few, the more pigment you add the more light you remove, in this case when you add yellow to a paper you are adding a pigment that absorbs all light except for the wavelength that corresponds to yellow light.
Red, green and blue work well because they match the color detectors in the human eye. Human eyes have red sensors, green sensors and blue sensors. Those are called cones.
– Red cones are sensitive to the end of the rainbow with reds and oranges.
– Green cones are sensitive to the middle of the rainbow, greens and yellows.
– Blue cones are sensitive to the blue end of the rainbow.
All of the cones overlap somewhat. For example, orange will trigger both the red cones a lot and the green cones a little. Your brain says “I can tell it’s orange because I’m getting a really strong signal on my red cones and also a moderate signal on my green cones.”
**Every color that humans can see is some combination of the signals from those red, green, and blue color-detecting cells.**
Some animals have more or fewer types of color detecting cells. They can detect colors that humans aren’t able to see. (I.e. two different shades that look identical to humans might look completely different to those animals.) If humans had different detectors sensitive to different colors; then red , green, and blue wouldn’t work as primary colors for us.
There are also sensors in the eye called rods, those pick up brightness only, but not color.
The short answer is that red, green, and blue are the colours the different cones (colour detectors) in our eyes are tuned to.
When we see something in-between red and green, both the red and green cones get triggered. So if we want to fake a colour in-between red and green, we use a bit of each.
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