Color

Color of Light

Color is the characteristic of human visual perception, which derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum, as a result of the way the object reflects or emits light.

Science of color - chromatics or colorimetry - studies the perception of color by the human eye and brain, the origin of color in materials, color theory in art, and the physics of light.

Visible Color

The photo-receptivity of other species varies considerably from that of humans. For example, bees are sensitive to ultraviolet but not to red. Some butterflies may have pentachromatic vision. Some shrimps have up to twelve dichromatic units.

Electromagnetic radiation is characterized by its wavelength (or frequency) and its intensity. Most light sources emit light at many different wavelengths.

Visible light for humans has wavelength within ~ 390 - 700 nm.

Colors visible to humans (with approximate wavelengths and frequencies):

  • Red  (700–635 nm, 430–480 THz)
  • Orange  (635–590 nm, 480–510 THz)
  • Yellow  (590–560 nm, 510–540 THz)
  • Green  (560–520 nm, 540–580 THz)
  • Cyan  (520–490 nm, 580–610 THz)
  • Blue  (490–450 nm, 610–670 THz)
  • Violet  (450–400 nm, 670–750 THz)

Colors of the rainbow - spectral or monochromatic - include all colors that can be produced by visible light of a single wavelength only.

Primaries

I use primary color hues in two groups of primaries:

  1. Pigment primaries:  magenta, yellow, cyan  (MYC), or  subtractive colour model CMY.
  2. Spectral primaries: red, green, blue (RGB), or additive color model.

Pigment MYC

 I use magenta-yellow-cyan primarily for color creation by mixing real pigments.

Magenta is an extra-spectral color - it is not a hue in monochromatic visible light, but a perception of reddish and wavelength blueish components. Magenta does not have wavelengths and simulated in the brain as red–purple. In digital graphics, combining magenta and complementary green on a black screen will create white. On the color wheel, magenta is between red and violet, which are at opposite ends of the visible spectrum.

Yellow is the color between orange and green on the spectrum. On screens, yellow is made by combining red and green at equal intensity. It is a complimentary color to true blue.

Cyan is made by mixing equal amounts of green and blue light on display. It is the complement of red and can be made by subtraction of red from white.

Spectral RGB

I use red-green-blue in adjusting or editing digital graphics.

The RGB color space corresponds to the three cone cell types that respond to three bands of light:

  1. Red - long wavelengths, peaking near 570 nm;
  2. Green - medium-wavelength, peaking near 540 nm;
  3. Blue - and short-wavelength light, near 430 nm.

Parameters

Alternative representations of the RGB color model, better aligned both with the computer graphics needs and the way human vision perceives color-making attributes:

  • HSV (Hue, Saturation, Value)
    • or HSL (Hue, Saturation, Lightness)
    • or HSB (Hue, Saturation, Brightness)
  1. Hue
  2. Saturation (chroma, intensity) - resembles making tints by adding white;
  3. Value (darkness, or Lightness in HSL model, or Brightness in HSB model) - resembles making tints by adding black or gray;

Intensity

The intensity of a spectral color relative to the context can considerably change its perception.

Color information is transmitted out of the eye by three opponent channels, each constructed from the raw output of the cones:

  1. red - green,
  2. blue - yellow,
  3. black - white (luminance).

This is why humans cannot perceive a reddish-green or yellowish-blue. Although in watercolor, I often divide the pigments of these colors in mixes by particles, to achieve such interesting feel of mixed color extremes.

The distinction rests on an important difference between the colours of light reaching our eyes from the various parts of an object and the colour we see as belonging to the object itself.
 - Dr. David Briggs, https://munsell.com/color-blog/difference-chroma-saturation