if black is all colors and white is no colors what is gray?
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The way your computer screen, phone screen, TV screen or basically any electronic dilevice shows an image is by producing light in either red, green, blue, or a combination of them. The standard uses a range from 0-255 for each of the three colors. RGB(0,0,255) is blue. RGB(0,255,255) is cyan. RGB(90,10,100) will be a deep, royal purple… Anyway, In this case, mixing all the colors together, RGB(255,255,255) is how they show white. And none of the colors RGB(0,0,0) is how they show black. Any time all three numbers are the same, the color is considered grayscale. So RGB(20,20,20) will be a dark grey and RGB(200,200,200) will be a light grey.
Light is much simpler than pigments. Pigments work by absorbing most light and reflecting a particular color. Since the incident light is typically white light, a combination of all colors, then pigments can be tuned to show any of them (technically the sun emits a bit better in yellow and it tails off either side, but the spectrum is broad enough we can treat it as white). You were taught in elementary school that red, yellow, and blue were the primary colors for pigments, which is close, but imperfect. A better set of primary colors is what your printer uses, CMYK stands for Cyan, Magenta, Yellow, Black. Each one absorbs one of red, green, or blue light and reflects primarily the third. Cyan as I said above is a combination of blue and green, so it absorbs red. Magenta is a mixture of red and blue light, do it absorbs green. Yellow is a combination of red and green (I know, that’s the hardest one to think about but it’s halfway between red and green on the rainbow, so it makes some sense) so it absorbs blue. And when I say ‘it absorbs’ I mean that pigment. Now, you can imagine how mixing yellow and cyan make green. Both of them reflect green, but cyan absorbs red and yellow absorbs blue. The only color left is green. That’s why you’re taught blue and yellow make green… I’m really not sure why elementary school thinks kids can’t handle the colors cyan and magenta…
So black, white, and gray. Yes, in this case, white pigments will reflect all colors together, and black will absorb all colors, reflecting none. So mixing all the pigments “should” make black, but this is an imperfect world where the pigments are all slightly preferential towards one or the other of their absorbed colors, leaving some brown sludge most likely. That’s why printers use that 4th cartridge of black toner. To print grey, all it does is use less toner. Either by making a thinner layer or by spreading out or shrinking the dots a bit more, depending on the technology behind the printer. I bet if you print out a grey scale image on a printer and look at it under a microscope, there will be millions of tiny black dots. The darker parts will have bigger and/or closer dots while lighter sections the opposite. Any part that is full black the toner may overlap, or just be the maximum amount designed to print. Any places that are white will just show the paper, no dots.
Most normal grey objects like rock or grey paint or cement should be considered a bad white (not an off white that’s kinda more yellowed). They reflect all colors equally, but not very well. They absorb some of them. To be clear, the whitest white thing you’ve ever seen in your life probably still absorbs a bit. So it could be replicated on a screen as RGB(254,254,254), but we can’t tell the difference. And similarly, if a rock reflects just a tad more blue, since this isn’t a perfect world, it might be replicated on screen as RGB(87,87,89) but they’re so close you can’t distinguish it from grey.
And one last note for you, light does not only come in three colors. The light emitted from the sun is a huge spectrum. We can only see wavelengths between about 450 to 780nm which form the rainbow from indigo to red. Yellow light can be either a mixture of red (780nm) and green(532nm) or it can be a pure yellow photon (≈580nm, just a guess). Our vision works on those primary three colors, red green and blue light. So we can be tricked by merging two colors like that. So when I say a rock reflects all colors equally and then say it is represented on screen by some RGB value, I really do mean it’s just a representation. Looking at the readout from a spectroscope would show you they are not identical, even though we were humans can’t tell the difference. This also means that some colors we see are not real photons of light that can exist. As I said, yellow is halfway between red and green, so the red cones and green cones in your retina both activate a little bit. (There is some overlap of the wavelengths that will activate them but they are labeled by the peak of that curve.) But since green is between red and blue, it’s impossible for a single photon to activate both red and blue without activating green. We would see magenta (or really, there are a whole house of colors including pretty much all shades and tints of pink, most purples, and up to the mauve/maroon index) that are only perceptible because of the way humans interpret colors. Photons of these colors cannot exist.
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