The HSI colour space (hue, saturation and intensity) attempts to produce a more intuitive representation of colour. The I axis represents the luminance information. The H and S axes are polar coordinates on the plane orthogonal to I. H is the angle, specified such that red is at zero, green at 120 degrees, and blue at 240 degrees. Hue thus represents what humans implicitly understand as colour. S is the magnitude of the colour vector projected in the plane orthogonal to I, and so represents the difference between pastel colours (low saturation) and vibrant colours (high saturation). The main drawback of this colour space is that hue is undefined if saturation is zero, making error propagation in transformations from the RGB colour space more complicated.
It should also be noted that, although the HSI colour space may be more intuitive, is not ``perceptual", in the sense that small displacements of equal size in different parts of the colour space will be perceived by human observers as changes of different magnitude. Attempts have been made to define such colour spaces: CIE-LAB and CIE-LUV are two examples.