For instance, the colorless 1 ct princess cut diamond engagement rings tend to be the most expensive when compared to the colored versions of the same ring.     Natural diamonds are available in a variety of colors, such as pink, blue, yellow, and red. These are known as fancy diamonds. Below are a few factors regarding the rarest and expensive colored diamonds. The Most Expensive Colors The most expensive diamonds are usually fancy colored ones. This is because of many reasons, the most important factor being the uniqueness of the diamond. This is related to how accessible a gem is. Besides, the color of the rock also determines the price of the diamond. Pink and red colored diamonds are the most expensive diamonds. Besides, lighter colored stones in the same category are usually more affordable than the darker varieties. The Rarest Diamond Colors Fancy colored diamonds are also very rare to find, which further accounts to their price. The rarest variety in colored diamonds is pink sapphire and rubies gemstones. Out of all diamonds, the Vivid variety of pink is considered the rarest.