Color is the presence of body color in a diamond. This is normally a yellowish body color that most often is not even seen unless compared against a truly colorless stone. This color is judged by grading a stone from the side against a stone of known quality.
The color grading scale runs from "D" or perfectly without color of any type on through Z, where the color is so bright that it is again considered to be desirable.
A number of things make this a difficult subject:

We are looking for a stone that has great light and beauty when it comes to the color. If it is to be put into platinum, it is probably more important for the stone to be of a higher color, G or better. This will keep the stone's color from contrasting with the color of the metal
If you are going to mount the stone in white gold, it is less important to have a very high color, as white gold is itself a yellowish white rather than a true white color. This is due to alloyed nickel and silver that is mixed with the yellow gold to give a whitish color. With platinum, alloyed white metal such as iridium is mixed with the white platinum.
If the stone is to be mounted in yellow metal and it is well made, then you can use a lower color and it will still look white by comparison. This allows someone on a budget to spend more on the clarity, cut, or carat weight of the stone. However, using a D color stone in yellow metal is quite beautiful and is of course a personal preference.
Our experience here at Winfields is that many of our customers actually like the warmer tint of yellow in their stones, and especially the slightly warmer dispersion that comes out of a K-L-M colored stone rather then the harder, colder dispersion that comes out of a colorless stone in the D-E-F color range. We have many stones of different colors, so don't hesitate to ask us to view our assortment of stones so you can best decide on the stone. If the "lower" colored rating is to your liking, you will find it comes with a lower price.