D,E, and F are considered completely colourless. This means that the stone will be colourless facing upward and also colourless out of it's setting facing downward.
G,H,I, and J are in the Near Colourless range and are also completely colourless face up. It is only out of their setting, face down against a white background that a trained diamond grader will see a hint of colour.
The grades below this will begin to show colour face up in their settings from grade K upward, becoming more noticeable further up the scale.