No property is more obvious to the casual observer of gemstones than their color. Color, however, is one of the worst properties to use to identify a gem. Though it may limit the possiblities and a few minerals such as malachite have a characteristic color, it is seldom enough for an identification.
A common problem with using color to identify minerals is that it is often quite variable for a particular mineral. Different varieties of quartz serve as an example; for instance, Amethyst is purple quartz, Citrine is yellow to brownish yellow quartz, Rose quartz is rose pink, Smokey quartz is a grayish variety, while milky quartz is generally opaque to translucent white, and Quartz crystal is colorless and transparent. With all these varieties and other minerals having similar colors, color alone is not adequate for a positive identification.
However, color can be valuable. Lists or determinative tables of minerals may be arranged by color, luster, hardness, etc. and can help you get started. The appendices of our textbooks have such lists and they have value.
One other problem with color is how subjective it can be. Do we usually agree on the name for a specific color? Though as far as identification of stones is concerned ane exact match is not important, it does have value for description of particular gems. Standards do exist, but it takes training and the purchasing of a set of standards to get started. The most famous set of standard colors is the Munsell Color Chart. The Munsell Chart (really a series of charts for soils, rocks, etc.) uses small squares of color as are used in paint stores. A special coding based on hue, saturation, and value is important to the system.
Though the Munsell system is objective, even here people may not totally
agree. The Gemological Society of America (WWW.GIA.edu) also had
its own color system for stones. This used plastic chips that are
compared to the stone.