Colorless diamonds are the ones that would fall in the GIA scale at the letters of D, E & F.  Colorless diamonds are some of the most rare with less than 1% of the diamonds that make it out of the earth being D or E.  What constitutes a colorless diamond, however, is not so much how that stone looks face up in a ring (of course the diamond is going to be "white"), but what really makes a diamond colorless is how it looks when it's observed face down against a flat white background.  A colorless diamond will still be white against the flat white background.   Here are the letters D, E and F alongside each other.

 

Here is a D alongside an E.  Note that there isn't any drastic difference between the two.  In real world observation the color difference is this minute as well.  D & E colors represent less than 1% of diamonds  that make it into engagement rings.

Here are the colorimeter results of these 2 diamonds.

 

Here is the D next to the F.  See you can barely note a difference.  Both diamonds face up are icy white and completely colorless.

As we progress further on in this section of "color," I'll be showing you some comparisons between the "D" next to other letters on the color scale.