Colors

Color Vision

Color is not a physical property of an object. It is the result, at the brain level, of the interaction between three elements: light, the object, and the observer. For the observer, the entire visual system is involved - all the way to the brain.

The Eye and How It Works Light rays pass through the cornea, then the pupil of the eye, and form an image of the object on the retina, the photosensitive layer at the back of the eye. The iris acts as a diaphragm, adjusting the amount of light entering the eye through contraction or dilation. The lens, whose curvature is adjusted by muscles, forms an inverted image of the object on the retina. The retina converts the received light energy into electrical energy, which is transmitted to the brain via the optic nerve. The brain (visual cortex) interprets these signals and reconstructs the image of the object.

The retina consists of two types of photoreceptors: rods and cones. Rods respond to shapes, even under very low light conditions; they allow for night vision. The pigment that reacts to light in rods is rhodopsin (or retinal purple). When the light intensity exceeds a certain threshold (daylight), rods become saturated and stop sending signals to the brain. At that point, cones become active for daytime vision and, specifically, color vision.

There are three types of cones, each with different sensitivity, which send three distinct signals to the brain:

• Hue perception
• Brightness perception (more or less intense light)
• Purity or saturation perception (more or less vivid or washed out)

The discovery by Helmholtz of these three types of cones and their associated sensations led researchers to use three-dimensional models to define color.

Maxwell had shown that most colors could be obtained by overlapping three separate beams of light. This was the principle of additive color mixing using 3 primary colors. It was Young, followed by Helmholtz, who proposed the trichromatic theory of vision, assuming our visual system includes three types of receptors.

We’ve seen that cones enable color vision. Each of the 3 types of cones contains a specific pigment sensitive to one of the 3 primary colors, with a distinct response to different wavelengths. Their peak sensitivities are approximately:

• 445 nm (blue-sensitive cones),
• 535 nm (green-sensitive cones),
• 570 nm (red-sensitive cones).

The Nature of Color

Color affects humans in physical, chemical, physiological, and psychological ways.

Color is a perception reconstructed by the brain-it’s not just what we see, but what we think we see.

Color also carries symbolic meanings across cultures and time, influencing art, religion (e.g., white or violet in Catholicism), architecture, and interior design (e.g., “warm” vs. “cool” colors).

Warm and Cool Colors

Some colors are described as “warm” (like red and orange) and others as “cool” (like blue and green). These qualities are widely used:

• In interior design: blue tones create a calming effect, while red energizes.
• In painting: warm-cool contrast creates depth and emotion.
• Even in medicine: chromotherapy uses color for therapeutic purposes.

Experiments have shown that people feel colder in blue-green rooms than in red-orange ones-even when the temperature is the same (between 12° and 15°C).

Blackbody and Temperature

Color is influenced by the light that illuminates it, so it’s essential to define light sources precisely.

Physicist Max Planck showed that the light spectrum of a blackbody depends only on its temperature. A blackbody is an object that absorbs all incoming radiation-similar to how incandescent bodies behave.

This principle is used to define light sources: A light source is characterized by the temperature of the blackbody that emits an equivalent spectrum. This temperature is called the color temperature (Tc) of the light source.

The CIE (International Commission on Illumination) defined 3 standard reference light sources for color measurement, using tungsten filament lamps with specific filters:

• 2850 K
• 4800 K
• 6500 K

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Kelvin (K) is the unit for measuring temperature. It uses the same scale as degrees Celsius (°C), but starts at -273°C, the theoretical absolute zero. Conversion:

$$K = °C + 273$$

For example, water boils at 373 K.

Color Wheel

The color wheel represents the 3 primary colors as well as the colors obtained by mixing them. Each color is placed between the two colors that generated it.

The three RGB primary colors are first placed at the vertices of an equilateral triangle.

Between each pair of primary colors, we place the resulting mixed colors: e.g.: blue + green = cyan

Complementary Colors

Two colors are said to be “complementary” if their mixture theoretically results in white-though in practice, it produces gray.

Each color has one and only one complementary color. There is, therefore, an infinite number of complementary color pairs. For the 3 primary colors of the RGB system:

• Magenta is complementary to green,
• Yellow is complementary to blue,
• Cyan is complementary to red.

Complementary colors are positioned directly opposite each other on the color wheel. When placed side by side, two complementary colors enhance each other. When mixed, they neutralize each other, producing gray.

Color Models

There is an infinite number of colors (computer monitors can now display over 16 million colors, for example). A purely sensory approach-such as that of a painter-is therefore not sufficient to objectively define a color.

This is what led to the development of colorimetry and color models. In this field, the CIE (International Commission on Illumination) has played a leading role for many years.

Several color models exist, each justified by its field of use or the technology involved:

• the RGB mode (red, green, blue)
• the CMYK mode (cyan, magenta, yellow, black)
• the HSL mode (hue, saturation, lightness)
• the CIE Lab mode
• the Duotone mode

It is also possible to choose a color directly from a “swatch”:

• indexed colors
• swatch libraries: Pantone, Trumatch, Focoltone, and others…

The RGB mode

A color is “primary” if it can’t be made by mixing other colors.

In the RGB system - used for screens and image software like Gimp - there are 3 primary colors: Red, Green, Blue.

Using these, you can create all other colors through additive mixing. For example, mixing red and blue gives magenta, a secondary color. By combining mixes, you can build a color wheel.

The CMYK Mode

Just like the RGB mode, the CMYK mode uses three “primary” colors to define any color. CMYK mode is used for printing.

The principle is to start with a white surface and subtract light using three primary colors: cyan, magenta, and yellow, as if using three color filters.

By overlaying these colors two by two, you obtain three new “secondary” colors: red, green, and blue. Overlaying all three colors produces black (which makes sense-since all colors have been subtracted!).

This is called “subtractive synthesis”, as opposed to the “additive synthesis” of the RGB mode. In fact, the primary colors of one mode are the secondary colors of the other.

Not all colors visible on screen in RGB mode can be printed using existing inks.

The HSL Mode

Another way to define colors, the HSL mode uses data that are more physiological than purely physical. It is, therefore, closer to how we perceive colors.

Based on the work of Munsell, the HSL color system also uses three characteristics to define a color:

• Hue,
• Saturation,
• Lightness.

Hue determines the color (the wavelength of reflected or transmitted light). It is represented on the circumference of the color wheel. Hue is expressed as a number indicating its angular position on the color wheel (starting at the top, moving clockwise). e.g.: red: 0°, green: 120°, magenta: 300°.

Saturation, or intensity, expresses the purity of the color (vivid, pastel, or washed out depending on the amount of gray in the color). Saturation is represented along the radius of the circle, as a percentage of purity: it is maximum on the circle (100%) and minimum at the center (0% = gray).

Lightness indicates whether the color is lighter or darker. In the HSL mode, all colors are represented inside a double cone. Lightness varies along the vertical axis of the double cone - the gray axis, from black at the bottom to white at the top. Lightness is expressed as a percentage: from 0% (black) to 100% (white).

An increase in lightness causes a color to shift toward white - just as a decrease in lightness causes it to shift toward black. A decrease in saturation (desaturated color) causes the color to shift toward gray (the axis of the double cone).