Only absolutely perfect diamonds are completely transparent, that doesn’t mean they’re the most valuable. Some diamonds have a very intense coloring that makes them far more unique than an ‘ordinary diamond’.
Diamond colours have a wide range from light blue to dark red. Of all the diamond colours red is the rarest and therefore also the most expensive kind. On the other side you have light yellow and light brown that are common 'flaws’ for transparent diamonds and are therefore seen as less valuable.