It's fair and true to say that you need an experienced eye to tell the difference between what would be described as the good commercial colour grades (approx G, H, I). However, there is no doubt that you would see the difference in a very white stone and a very "off white" stone. For example a D/E colour and an I/J colour.
Let me explain briefly the colours. This is in the white series and does not refer to Natural 'Fancy' coloured diamonds. For example yellows, pinks and even more rarely blue. But I will come to those later.
In descending order, from the whitest becoming more and more 'off' white. D, E, F (top white, bright, crisp sparkly crystal) G, H (still very nice, but lack that pure white crystal appearance) I, J, K,L (obvious hint of yellow) and M, N (this is where the Fancy yellows start! (For a more defined explanation please see GIA colour grades)
The difference in the colour is absolutely tiny, the best way to see it is by comparison so that the difference is made more prominent. It is, however still challenging to see when not viewing diamonds everyday.
Naturally 'Fancy' coloured diamonds are an entirely different ball game. I am a diamond grader and have over 10 years experience in the gemstone trade but I would not attempt to grade or value one of these.
Most have a main colour with a mix of other colour or tone. Trust me it's very complex and very very expensive. I have access to these stones, nothing is out of the question but very deep pockets are required. From tens of thousands of pounds upwards, this is not for the faint hearted!!
Fancy coloured diamonds are special and a true marvel of the natural world. Their rarity and demand leads to very high prices. If your curious see Christies and Sotheby's both of whom have sold many of them, all be it exceptional examples.
The colour in these natural diamonds is sometimes due to an impurity in the atomic structure, for example nitrogen in the yellow diamonds and boron in the blue diamonds. It can also be due to something called 'plastic deformation,' basically a fault in the atomic structure of the diamond. It's complicated and I'm no physicist, but it is fascinating!!
They are not, however, to be confused with Colour treated diamonds. Diamonds which have been heated and/or irradiated to induce colour. The tones that you get are mostly very specific and very different to their natural counterpart. As is their price, these treated stones are quite simply astronomically cheaper. Although they can add an unusual flash of colour for a fraction of the price, my personal stance is that I would not invest in these stones.
As always if you have any questions or need some advice please don't hesitate to get in contact. Please take great caution when investing in fancy coloured diamonds!