Colour

Colour attributes

Each colour has its own distinct appearance, based on three elements: hue, chroma and value (lightness). By describing a colour using these three attributes, you can accurately identify a particular colour and distinguish it from any other.

Hue : Hue indicates a particular colour sensation which is dependent simply on the relevant wavelength; the inherent colour of a thing; the purest or brightest form of a colour having no white or black mixed with it. Quite simply, hue is how we perceive an object’s colour — red, orange, green, blue, etc. Each hue has an intrinsic tonal value on the chromatic scale.

Chroma: Chroma is the extent of a colour’s brightness or saturation; the purity of a colour, in other words, how close the colour is to either gray or the pure hue. For example, think of the appearance of a tomato and a radish. The red of the tomato is vivid, while the radish appears duller.

Lightness: Lightness is the luminous intensity of a colour. It is that attribute of a colour which indicates the extent of the light which it reflects. Colours can be classified as light or dark when comparing their value.

Colour and Colour Mixing

A coloured surface appears the coloured because light is reflected from it in that colour's wavelengths only. All other wavelengths predominantly are absorbed by it. A blue object reflects blue light but absorbs most of the red, orange, yellow, green and violet. Black and white is slightly different. They are not, strictly speaking, colours. White surfaces reflect all or nearly all wavelengths and black surfaces absorb them totally. Response curves for the rods (colour sensitive photoreceptors of the human eye). The peak response values occur at 580 nm for red, 540 NM for green, and 440 NM for blue. There is no excitation of the blue cones above 550 NM .The overall response of all three cones together is at 560 NM (yellow-green).

Additive Colour Mixing
The absence of light is darkness, add light to it
Superposition (lamp overlap)
Rapid alternation (biased LED) "persistence of vision"
Small elements (TV pixels, halftones)

The basic rules of additive colour mixing.
red + green = yellow;green + blue = cyan
blue + red = magenta ; red + green + blue = white

Subtractive colour mixing
Subtractive colours are produced when white light falls on a Coloured surface and is partially reflected. The reflected light reaching the human eye produces the sensation of colour. Subtractive colour is based on the three colours Cyan, Magenta and Yellow. Varying the mixture of these primary colours produces other colours. When these three colours are mixed together at 100% they produce black. The absence of CMY pigments would result in white.

Metamerism :
When two object with different spectral graphs present identical coloured appearance in specific lighting and observation conditions, and different appearance when lighting or observation conditions are changed ,then metamerism is said to be in existence. Metameric index (D65 ~ A) = { (LD-LA)2 + (a*D65-a*A)2 +( b*D65 - b*A)2 }1/2