Colour is the second most important indication of the stone’s appearance as it affects how white and clear a diamond looks. With a poor colour grade the stone can appear slightly yellow instead of the desired brilliant white.
Another important thing to look out for is the fluorescence. It might not be as valuable on the open market as it sometimes reduces the price and can dull a stone. But sometimes it also can enhance colour and make the stone look brighter than it is.
Every natural diamond has a a slight tint of yellow. The closer to being “colourless” a diamond is, the rarer it is.