The GIA Color Scale is the most widely used diamond color-grading system in the world, and the normal color range of diamonds is often called the “D-to-Z scale.”  The letters in the D-to-Z scale don’t describe actual colors, or hues.  Each letter actually represents a range of color that is based on a combination of tone (darkness or lightness) and saturation (intensity).  The combination is called depth of color, and it is a measure of how noticeable a color is.  Diamonds can differ slightly in their depth of color, but still be assigned the same color grade. 

Color is easier to see in larger stones than in smaller ones.  In weights under 0.25 carat, only trained graders can distinguish between diamonds in the D-to-F range; and then only if the stones are not in a setting.

When mounted, G, H, and I diamonds might look colorless face-up.  That is why most gemological laboratories don’t grade mounted stones.  Part of the difficulty in color-grading mounted diamonds is that the mounting affects the apparent color by covering part of the stone. 

Another factor that affects a diamond’s apparent color is the color of the mounting itself.  Yellow gold makes slight amounts of yellow or brown less obvious, while white metal mountings make the color in yellow or brown stones more apparent.  On the other hand, white metals enhance the appearance of colorless stones. 

Face-up and set in yellow gold, J, K, and L diamonds that weigh less than 0.50 ct. look nearly colorless to the untrained eye.  The color is easy to see in larger diamonds or when the setting is white metal.  Beyond M, most consumers can see color pretty easily.  As the grade gets closer to the Z, the color is obvious even in small diamonds.