Different flowers have same colour
I think parallel evolution is likely due to the cost to produce the pigments in plants.
Xanthophylls and carotenes are made from only carbon, oxygen and hydrogen. This provides most of the yellow colours without using up nutrients.
Anthocyanins and betalains also need nitrogen, which can be a limiting nutrient for plants.
Wikipedia has a section on plant pigments at:
http://en.wikipedia.org/wiki/Biological_pigment