Are pink diamonds or yellow diamonds more expensive?

Natural pink diamonds are more rare than natural yellow diamonds, probably because the common trace element nitrogen is responsible for a yellow cast in a diamond. Some colourless or 'white' diamonds are considered yellow, when graded with a colour grade from I to Z, as below.

Natural pink diamonds exist 'due to structural anomalies" according to Wikipedia, and are very rare.

Some diamonds can be altered (or enhanced) to appear yellow or pink, and when altered, are less valuable than naturally colored diamonds.