Simply put, the less color there is in a diamond the more value it has. There are two reason for this.
#1 Colorless diamonds are quite rare.
#2 The brilliant cut was designed to eliminate the time that light travels through a diamond thus keeping the diamond's colorless qualities.
How Diamonds Are Graded
Diamonds are graded on a scale from D-Z based on how much color is in the diamond. The letters A, B, and C are reserved just in case diamonds are discovered that are more colorless. Perhaps it's a twisted joke because diamonds start with D. Until thirty years ago, a diamond would be examined by experts under microscopes to get the diamond color grade. Now most diamonds are graded by a machine which increases reliability.
Visible Color Range
To 95% of the population, D-G diamonds are indistinguishable to the naked eye. H-L have some yellow visible but can be made less visible by a good jeweler. When I mean some yellow, I'm not talking about school bus yellow, I am talking about the ever slight hint that the diamond might not be 100% colorless to the naked eye. They are still GREAT diamonds which we sell a lot of.
Best Color Range
We are asked daily, "What color should I buy?". The only true answer is that it entirely depends on what you want to do. Some people feel a burning in their soul to only buy D color diamonds. Some want to stretch their budget and get the biggest diamond that doesn't look yellow. If really pressed, I tell customers about my wife's J color diamond, which she gets compliments all the time.