I would say that Green Diamonds are next. Of course it is the pure green colors that are the most valuable, colors like Fancy Intense Green and Fancy Vivid Green.
The jewelry grade (white) diamond that is most valuable is identified as a D-colour stone. In fancy coloured stones, both blue and pink stones are valuable, although red might be the most rare, and therefore the most valuable. See the link, below.
Blue diamonds are rarer and more valuable than red diamonds. The presence of the boron element in blue diamonds gives them their color, while red diamonds get their color from structural defects in the crystal lattice. Both colors are extremely rare and highly prized by collectors and investors.
Red
The color is red!
red
white and red
Red.
The Kazanjian Diamond is red. You can read more about it, below.
The rarest natural diamond colours are probably red, green and blue. If you can find a large stone in one of those colours graded as flawless or very slightly included with a cut that shows off the stone's colour and high clarity, you will probably pay the most per carat for this stone.
Red and green are contrasting colours, and green next to red stands out.
The color of the snake is reddish and blends in to the color or the sand in its environment.
The rarest diamond color is the natural red diamond. Most diamond dealers have never seen one, other than a "red" diamond that was originally congac or yellow color, and was treated to look red. The second rarest is the natural untreated green diamond, then, the natural untreated orange, then the blue and the pink diamonds. The value depends on many other factors however. But colorless diamonds are still the most sought after.