When we discuss the colour of a diamond, actually we are discussing the colourlessness of a diamond as a diamond with more colourlessness is considered a pure diamond. Normally, most diamonds are white; however some are yellowish to brown or silver to grey.
Only those diamonds are colourless which are composed of 100% pure carbon without any impurities or scums. In a white diamond, the colour impurity is caused by persistent traces of some elements like nitrogen, boron, hydrogen and few more. Nitrogen traces create pale yellowish or brownish tints in the diamond. The presence of tint in a white diamond is not considered a good sign.
A diamond with more whiter or clearer colour is considered more valuable.
There were so many diamond colour grading system was available in earlier times which were far away from coming up with precise grading results. To name a few were Alphabet, Arabic and Roman. The Gemological Institute of America (GIA) came up with a grading system with specific guidelines to grade diamond colour.
In this method, a diamond is placed table-down, pavilion-up and are checked with a 10X loupe in a controlled light by a person who is expert in diamond colour grading. Diamond colour distinctions are so elusive that only skilled eyes will be able to view it. However, these small distinctions make a huge difference while deciding the price and quality of a diamond.
There are diamonds available that do not fall into the GIA grading colour category. These diamonds are referred as a fancy Coloured diamonds. These diamonds generally have a variety of colors like pink, canary yellow, blue, green and brown.
Some of these fancy diamonds are considered more expensive and valuable than colourless or white diamonds. Fancy diamonds like pink, canary blue and green diamonds are rarely seen and intense which are priced more than white or colourless diamonds.
Fancy diamonds are generally termed as Light, Fancy Light, Fancy, Fancy Intense, Fancy Deep, Fancy Dark and Fancy Vivid.
Fluorescence can also affect the colour of a diamond. Some people enjoy this effect while some do not. According to the most gemologists, this effect is not preferable in the diamond.
The fluorescence effect can be seen in some diamonds when they are exposed to long-wave ultraviolet lights such as disco lights. However, this cannot be detectable by eye. This effect will alter the colour of a diamond very often. The colour changes of a diamond depends on the colour of fluorescence light. A Fluorescence of blue light will make yellow Coloured diamond look whiter. However, in some cases, it also causes a stone to appear more milky or oily. This is called an "Over Blue" effect.