Most modern TV/computer screens don't have so saturated RGB primary colors to name any particular triple wavelengths as the wavelengths of a particular monitor. Moreover, if we try to ascribe some wavelength to a primary color by applying the concept of dominant wavelength, we'll still have too much variation in actual RGB primaries in different screen technologies. Show The most common color space to which the computer monitors (and, to a lesser extent, TV screens) adhere is sRGB. There we have dominant wavelengths of about 549 nm for green, 612 nm for red and 464 nm for blue with respect to the sRGB white point. On the other hand, there are many displays like OLED TVs and smartphone AMOLED screens, which use another (more saturated) set of primary colors, e.g. Display P3. In this color space the dominant wavelengths are close to those of sRGB, but somewhat different. And even among the displays which nominally correspond to the same color space standard, there's quite a noticeable variation of primary colors and the proportions in which they are mixed for any given RGB value. So if you have two monitors (especially of different makes or models) and try to display the same color on them, you'll notice that the colors don't match. Color calibration is a procedure with which we can try to minimize differences of the colors displayed by a monitor from the reference color space, but if monitor's primaries are too far from the nominal, this won't lead to too good results, so some residual mismatching will still remain.
We all remember our first set of paints: red paint plus green paint do not make yellow! Yes, there's a big difference: with paints we are not adding light, but subtracting it. White paper reflects all colours; when we add coloured paint to it, that paint allows some colours to pass through, but absorbs (subtracts) others. Just as most colours can be made by mixing light with proportions of the three additive primaries, we can also produce most colours by starting with white light and subtracting different proportions of three subtractive primaries: The subtractive primaries are these:
We hope you've noticed that the subtractive primaries are the complementary colours of the additive primaries. In the diagrams below, we represent white light as made of red + green + blue, because it stimulates our red, green and blue photoreceptors. In practice, you are probably reading this on an RGB monitor, then the white really is made from an RGB pixel combination.
We can expand those in still diagrams: white light is represented in the top row: it then passes through each of the subtractive primary filters to give the colour in the bottom row: Magenta filter
Cyan filter
Yellow filter
Subtractive colour mixingWhat do we get if we add magenta + cyan paint? We can extend these diagrams to answer that question.
Or, if you prefer tables to the animation above: Yellow filter + cyan filter
Yellow filter + magenta filter
Magenta filter + cyan filter
Again, we can show this in a still diagram: Cyan filter + yellow filter + magenta filter
However, this is not really subtractive mixing: if you are reading this on a monitor, then all the colours and white are made from different proportions of red, green and blue. If you want to see real subtractive mixing, you could print it out. Your printer will start with white paper and then print a mixture of cyan, yellow, magenta and black pigments. There are two reasons for the black pigment in printers: first, a mixture of CYM can, with adjustment, give reasonable dark grey. Also, black pigment is usually cheaper than a CYM mixture.
3, 4, 5, 6, 7 or many colours?Three additive and three subtractive primaries? 6 in total? Then what happened to the ROYGBIV rainbow that we learned at school? While the three primaries are (for humans, at least) a logical set, divisions of the spectrum are arbitrary. As for much else, it appears that we have Newton to thank for the 7-coloured rainbow. Fisher (2015) writes "The medieval rainbow had just five colours: red, yellow, green, blue and violet. Newton added two more — orange and indigo — so that the colours would be “divided after the manner of a Musical Chord” (I. Newton in Opticks 4th edn, 127 (William Innys), 1730)". In several of the figures above, the (varying) colour is indicated by a hexadecimal number, such as ff0000. The first two digits give the hexadecimal proportion of red, the next to green, the final two blue (pause the animations and check that 00ff00 is green, 0000ff is blue, and ffff00 is yellow). Hexadecimal means that each digit counts successively 1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,0. So 10 in hexadecimal is 16 in decimal, and 100 in hexadecimal is 162 = 256. So this numbering system can label 2563 = 16,777,216 colours, which is probably enough even for an interior decorator. Many graphics packages allow you to specify colours by with six digit hexadecimals (or sometimes six decimal digits), so you can experiment with these. (Incidentally, Newton's choice of 'orange' as a name was a good one. It is now echoed by such colours as apricot, peach, avocado etc.) The following link takes you back to the multimedia tutorials The Nature of Light and The Eye and Colour Vision. |