When the relative rather than the absolute values of a variable are important, gradual color changes as a tonal code should be used to show the relative values of a single variable.
Additional Information:
For example, in displaying tank depth, a saturated blue might be used to show the deepest point, with gradually desaturated blues to show decreasing depth. Gradual color changes should not be used when absolute values are important, or to code data into discrete categories. For example, gradual color changes should not be used to indicate the level of a storage tank as it is drained or filled. Instead, a set of discrete codes indicating dangerous and acceptable levels may be more appropriate.