Diamond colours are usually measured using the GIA’s Grading Scale. This involves a D-Z scale, with D signifying a totally colourless or white quality (typically the most expensive), and Z giving a slightly yellow hue.
The simplest and most effective way of assessing diamond colour is to check it from the side, unmounted while positioned against a white background, wielding a daylight fluorescent bulb.
Experts generally view standard diamond quality as falling into the D-J range. The shape of the diamond also affects its position on the colour scale. For instance, a round brilliant diamond conceals colour extremely well, which means you can select a stone that is ranked way further down the scale without spotting any yellowing.
However, more elongated diamond shapes, including the oval cut, show colour much more easily. Always remember that diamond colour is a matter of personal taste, above all, and it doesn’t bear any relation to the quality of the stone.
With round and emerald cuts, it is possible to go as low as a J grade without finding any obvious colour. By contrast, for cuts like oval, radiant, pear, princess, marquise, you should be looking for a slightly higher ranking (G and up) to find colourless stones.