USDA/AMS is the most general source for visual color standards for fruits and vegetables and offers a catalogue of color comparators and equipment they have available.
USDA Equipment Catalog for Fresh and Processed Products Inspections is available on the internet here and provided through:
Processed Products Branch
U.S. Department of Agriculture
P.O. Box 96456, Rm. 0709, So. Bldg.
Washington, D.C. 20090-6456
USDA Visual Color Standards and other equipment related to fruits and vegetables can be ordered from: