However, they often mistake mounted K,L,M diamonds weighing less than half a carat as being colorless when they are mounted in white metal. Although yellows are more easily identified by consumers, brown and gray tones are also graded on the same scale. When used properly under controlled lighting, such as a GIA Diamondlite, master stones provide a reliable method of comparing color in unmounted diamonds. Jewelry store halogen lighting does not provide a good environment for accurate color grading. They do that in the back of the store under controlled lighting conditions.
Clothing stores do the same thing.
Another B.S. stunt is suggesting that printers ink is the same as diamond color. The poster is designed to demonstrate how subtle the color differences are. And they'd let you. A really good salesperson will even take a moment to explain fluorescence to you and show you what it looks like under the Diamondlite. If a diamond is mounted, it will be impossible for you as an inexperienced grader to determine it's true color.