There are certain parallels 
worthy of mention between the 
perception of lightness and the 
perception of chromatic hues—–
the colors of the visible 
spectrum. Color constancy 
occurs if light of predominantly 
one frequency is used as the 
source of illumination. Then, a 
colored region will no longer 
reflect light to the eye of the 
same frequency that it would 
when receiving white light. 
Nonetheless the region will 
continue to appear in its 
normal color. Just as in the 
case of lightness constancy, 
the explanation concerns the 
fact that the chromatic 
illumination also falls on the 
area surrounding the colored 
region so that the relation 
between the two is unchanged 
in certain respects. There is 
also a parallel with respect to 
the shades and tints of colors. 
Colors are said to vary along 
three dimensions: the hue 
itself (based on the frequency 
of the light), the saturation or 
pureness of color of a given hue 
(based on the extent to which it 
is mixed with white light), and 
the degree of lightness (based 
on the lightness or darkness of 
that white-light mixture). If 
lightness is based on ratio, and 
it seems that it is, then the 
appearance of a color should be 
affected by variations in the 
lightness or darkness of a 
neighboring or surrounding 
region. For example, an orange 
disk can be made to appear 
brown simply by increasing the 
intensity of white light in a 
surrounding ring. Few people 
realize that brown is simply a 
shade of orange.