The highest grades in Color start with D. The more white a Diamond is, the more rare and valuable it is. The more Nitrogen element is in the mines of the stones, the more yellow the Diamonds will have in it. D, E and F color are very white and very bright. There will be no tint of color in these diamonds, and very hard to tell the difference between these colors.
The next section in Color is Near Colorless ranging from G through J. With the naked eye, these colors are harder to tell the difference. And even harder when a stone is set into a setting.
Sometimes, with GIA Certified stones, the K color grading is sufficient for specific uses for example earrings, or an Engagement Ring using a Yellow Gold or Rose Gold setting. Most of the time however, we stay away from this color section, especially anything lower than K. This section will always have a tint of yellow even in natural light.
These stones are considered the lowest of the White color grading scale. However, they are often used for the "Fancy Yellow" Diamond Engagement Rings or other Jewelry. This section is actually very close to the "Fancy Light Yellow" grading scale, and with the 18k Yellow Gold cup that holds Yellow stones, the color is much more bright and Yellow.