Hue

Hue, along with saturation and brightness make up the three distinct attributes of color. The terms "red" and "blue" are primarily describing hue - hue is related to wavelength for spectral colors. It is convenient to arrange the saturated hues around a Newton Color Circle. Starting from red and proceeding clockwise around the circle below to blue proceeds from long to shorter wavelengths. However it shows that not all hues can be represented by spectral colors since there is no single wavelength of light which has the magenta hue - it may be produced by an equal mixture of red and blue.

	There are many different mixtures of wavelengths which can produce the same perceived hue. The achromatic line from black to gray to white through the center of the circle represents light which has no hue.

The distribution of hue around a circle is used in commercially available paint and pigment mixing guides like the Color Wheel.

Saturation

Saturation, along with hue and brightness make up the three distinct attributes of color. Pink may be thought of as having the same hue as red but being less saturated. A fully saturated color is one with no mixture of white. A spectral color consisting of only one wavelength is fully saturated, but one can have a fully saturated magenta which is not a spectral color. Quantifying the perception of saturation must take into account the fact that some spectral colors are perceived to be more saturated than others. For example, monochromatic reds and violets are perceived to be more saturated than monochromatic yellows. There are also more perceptably different levels of saturation for some hues - a fact accounted for in the Munsell color system

	There are many different mixtures of wavelengths which can produce the same perceived hue. The achromatic line from black to gray to white through the center of the circle represents light which has no hue.

Note that the blue of the sky is more saturated when you look further from the sun. The almost white scattering near the sun can be attributed to Mie scattering, which is not very wavelength dependent. The mixture of white light with the blue gives a less saturated blue.

One of the features of the commercially available Color Wheel is to help visualize the effect of adding white paint or pigment. It makes the color less saturated as shown in the image below.

Brightness

Brightness, along with saturation and hue make up the three distinct attributes of color. The brightness of a colored surface depends upon the illuminance and upon its reflectivity. Since the perceived brightness is not linearly proportional to the reflectivity, a scale from 0 to 10 is used to represent perceived brightness in color measurement systems like the Munsell system. It is found that equal surfaces with differing spectral characteristics but which emit the same number of lumens will be perceived to be equally bright.

	If one surface emits more lumens, it will be perceived to be brighter in a logarithmic relationship which yields a constant increase in brightness of about 1.5 units with each doubling of brightness.

The images above from the commercially available Color Wheel depict 10 levels of brightness for gray or achromatic light.