Additive Mixing Of Colored Light

Additive mixing is what happens when different colored lights illuminate a reflective object, like a white wall. The white wall does a good job of reflecting back the same color of light that you shine on it, so the results will be very much like what you get when you shine more than one color light straight into your eye.

Subtractive Mixing Of Paints, Pigments, And Filters

Consider a White light (which can be thought of as Red + Green + Blue), shining on a Yellow ball. Since your brain creates the sensation of Yellow, your eyes must be getting Red + Green. So Red + Green + Blue shine on the ball and Red + Green are reflected to your eye - the Yellow ball must be absorbing the Blue. The ball isn't really Yellow. The ball is anti-Blue, which we see as Yellow.

Thus a coat of colored paint is like a bad mirror, absorbing parts of the spectrum and reflecting the rest.

We'll look at how we can mix unit quantities of each color. We will use a negative notation to indicate what paint does to light reflecting off it, but this isn't exactly mathematical in that we don't have any negative result. When we say "Cyan is negative Red", we mean "Cyan fails to reflect Red, if it were present."

• The primary colors in paint-mixing land are backwards from light-mixing land:

  paint "color"	what paint does	light to eye	brain perceives
  Cyan	-Red	Blue + Green	Cyan
  Magenta	-Green	Red + Blue	Magenta
  Yellow	-Blue	Red + Green	Yellow

• Mixing two primary colors in paint-mixing land:

  paint "color"	what paint does	light to eye	brain perceives
  Cyan + Magenta	-Red -Green	Blue	Blue
  Cyan + Yellow	-Red -Blue	Green	Green
  Yellow + Magenta	-Blue -Green	Red	Red

• Mixing three primary colors in paint-mixing land:

  paint "color"	what paint does	light to eye	brain perceives
  Cyan + Magenta + Yellow	-Red -Green -Blue	none	Black

Please note that when you put a filter in the path of a light beam, you are engaging in subtractive color mixing - the filter keeps certain components of light from getting through. But if you have two filtered lighting fixtures and shine both lights on the same thing, the two lights are mixed additively.

Subtractive color mixing, as if you were to pour puddles of three primary colored paint on a surface, with perfect blending where they overlap.

Additive And Subtractive Mixing - Colored Lights On Colored Surfaces

• White light preserves the color imparted by the paint. White paint preserves the color imparted by the light.

  light "color"	paint "color"	what light contains	what paint does	light to eye	brain perceives
  White	Cyan	Red + Green + Blue	-Red	Green + Blue	Cyan
  Magenta	White	Red + Blue	reflects all	Red + Blue	Magenta

• If the paint absorbs all the light, you get black

  light "color"	paint "color"	what light contains	what paint does	light to eye	brain perceives
  Green	Magenta	Green	-Green	nothing	Black
  Red	Blue	Red	-Green - Red	nothing	Black

• Other examples:

  light "color"	paint "color"	what light contains	what paint does	light to eye	brain perceives
  Yellow	Yellow	Red + Green	-Blue	Red + Green	Yellow
  Yellow	Cyan	Red + Green	-Red	Green	Green
  Yellow	Green	Red + Green	-Red -Blue	Green	Green

These last two examples are interesting, so I'll repeat:

• Yellow light on Cyan paint looks Green.
• Yellow light on Green paint looks Green.

This is also why it is hard to pick out your car under the sodium lamps that are used to illuminate parking lots: the colors don't look the same as during the day, and several different colors often look the same.

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