A diamond's color is a measure of the stone's whiteness, meaning it is actually measuring the lack of color a diamond has.
The American Gem Society (AGS) has a scale to measure a diamond's color, ranging from 0.0 (colorless) to 10 (light).
For most people, the difference between colorless, near colorless, and even the highest range of faint yellow diamonds can be hard to see. It's actually possible for a diamond to go above a "10" color, but at that point the diamond is re-categorized as a "Fancy Color Diamond." These extremely rare diamonds exhibit intense colors such as blue, pink, yellow green or even red and have their own set of grading parameters.
Use the color comparison tool below to see the difference between the various grades of diamond color.
The video below covers the basics of diamond color.