Relative purity or strength of the spectral color of soil is

The relative purity or strength of the spectral colour of soil is Hue. The concept of a colour system with a hue was explored as early as 1830, and Munsell's colour system was a great step forward in realizing perceptual uniformity.
It is one of the main properties of a colour, which refers to the degree to which a stimulus can be described as similar to or different from stimuli that are described as red, orange, yellow, green, blue and violet (colour of vision).