They can also be unintended consequences in the evolution of the flowers. As I understand it, they are beautiful because they are trying to attract the attention of their pollinators.
Some flowers are plain-looking because they are beautiful in infrared or ultraviolet, for example. That is, they are beautiful to their intended non-human audience. The fact that other flowers are beautiful also in the human visible spectrum is either a fortunate accident for us (if it is a wild flower) or a deliberate choice by their pollinators (if it is a domesticated flower that people plant on purpose).
Good point about coevolution. Of course all thoughts on this subject are (currently) speculation.
Here's a quote by Georgia O'Keefe: “Whether the flower or the color is the focus I do not know. I do know the flower is painted large to convey my experience with the flower – and what is my experience if it is not the color?” I don't think color is the whole story: As I write this I have a vase of yellow tulips in front of me. My wife and I are particularly fond of this particular combination, any other flower of the same yellow color won't do.
Some flowers are plain-looking because they are beautiful in infrared or ultraviolet, for example. That is, they are beautiful to their intended non-human audience. The fact that other flowers are beautiful also in the human visible spectrum is either a fortunate accident for us (if it is a wild flower) or a deliberate choice by their pollinators (if it is a domesticated flower that people plant on purpose).