This refers to the hue and the purity of the hue. Lets take yellow as an example. The color of a fancy yellow diamond might be described as yellow, orangish-yellow, brownish-yellow, brown-yellow, and so on. Understanding the differences in the wording is very important. The final word in the color description is the hue; the word or words preceding it are the modifiers of the hue.
Having no modifier means the color is a pure hue and, depending upon the color, can be very rare. Some color combinations are rarer and costlier than others. For example, lets consider two diamonds where one is orangy-yellow and the other is brownish-yellow. In both cases, the hue is yellow but since orange is rarer than brown, the orangy-yellow stone would be more valuable than the brownish-yellow stone.
Now lets look again at the description of the brownish-yellow stone. Here the primary color is yellow, with a lesser degree of brown modifying the color. However, if the words were reversed, that is, yellowish-brown rather than brownish-yellow, the words would tell us the diamond is brown with some lesser amount of yellow modifying the color. Again, since brown is a much less rare color, a yellowish-brown diamond would have less value than a brownish-yellow.
For more information on diamonds read DoWs4Cs on fancy-colored diamonds information. More