The GIA describes color as follows:
It is worth noting that two diamonds with identical GIA color grades will probably not face up the same to the naked eye because each unique diamond shows color and brilliance a little differently.
Also interesting is that the same diamond submitted to the GIA for color grading more than once will not necessarily receive the same grade each time.
Color is graded on a spectrum and determining a specific grade is inevitably subjective [within limits]. It is therefore understandable for a color grade to be seen slightly differently at different times.
Diamonds from D to H color do not show any yellow tint; I to K colors show just a hint of color and when cut perfectly often provide extraordinary value.
Visit an Ascot showroom to learn more.