xanthometer

from The Century Dictionary.

• noun A graduated scale of colors from blue to yellow, white, or green, for determining the color of ocean or lake water. Well-known examples are Forel's scale from blue to olive, for use in the lake of Geneva; O. Krümmel's modification, to which the name ‘xanthometer’ was first given, for use on the plankton expedition of 1890; and the scale of W. Ule of Halle, who extended the scale to 21 numbers.