color mixing confusions

Discussion in 'Beyond the Basics' started by kkamin, Apr 13, 2010.

  1. kkamin

    kkamin TPF Noob!

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    Hello,

    I understand that RGB is an additive process (emitted light) that combines red, green and blue to produce a wide gamut of colors.

    I understand that printing uses a subtractive process (reflected light) that uses CMYK: cyan, magenta, yellow and black to produce a wide gamut of colors.

    ------------------------------------------------------------------------

    ok,

    now why...

    •do painters use red, yellow and blue? I'm not so much concerned about them using yellow instead of green, but why are they using additive colors when clearly they are in a subtractive color medium.

    •The color spectrum is made up of red, yellow, orange, green, blue and violet. Where did cyan and magenta go? wtf is up with orange? What is happening?

    •In Kelvin scale we use for color balancing, what colors are contained within the 'cool' and 'warm' color temperature casts? When I am in Adobe Camera Raw, I am given two sets of sliders to correct the white balance. One controls the color temperature. The other a GREEN and MAGENTA cast. Does the Kelvin color temperature slider contain red and cyan as well as blue and yellow? For example, if I shoot at 8000K under 5500K lights, what warmer colors am I introducing into the image? Is it yellow and red?

    Thanks for reading.
     
  2. KenC

    KenC Been spending a lot of time on here!

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    "•do painters use red, yellow and blue? I'm not so much concerned about them using yellow instead of green, but why are they using additive colors when clearly they are in a subtractive color medium."

    The reason is that when one is dealing with pigments/dyes as opposed to light, there is no way to get yellow from any other color, so it has to be a primary. Red and blue (not the same blue as in RGB - more a royal blue than navy) are probably just conveniently similar to common pigments because you could vary them and still mix to produce pretty much any color. The other colors are missing from the set because you only need three to establish the color space. Of course painters would more likely use an orange paint than mix it from red and yellow.
     
  3. Garbz

    Garbz No longer a newbie, moving up!

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    From what I've gathered, gamut. I remember being told at some point CMY were chosen for printing because they are more similar and more easily controlled. I.e. to make "black" you can either add Blue, Red, and a ****load of yellow, or you could add roughly equal parts cyan, magenta and yellow.

    I have no idea how true this is.

    But for digital processing you need to learn the idea of opposite colours. Think of the curves tool in photoshop. With a single curve you could for instance add or reduce the amount of green in an image. But wait are you actually adding green, or removing magenta?

    White balance doesn't fit nicely under adding or removing colours. It can be better visualised in the spectrum as a black body curve. The relative weights of red green and blue are then also taken into account.

    For instance to know how warm or cold something is you basically need to work out the intersection between
    [​IMG] and [​IMG]

    Don't under-estimate just how much maths is involved in colour. It definitely isn't as easy as raising the colour temperature adds more or yellow.
     

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