The reason that flowers come in so many different shapes and colours is so that they are able to fertilize.
Flowers rely on insects for pollination but it fertilizes only when an insect carries the pollen from the same plant. The shape of the flower ensures that only a certain kind of insect can pollinate it. Flat flowers like sunflower, Roses, etc. are visited by bees, whereas flowers that are tube shaped attract only insects which have long tongues.
Below Flowers info from Wikipedia (Read More):-
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Magnoliophyta, also called angiosperms). The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds. The process begins with pollination, is followed by fertilization, leading to the formation and dispersal of the seeds. For the higher plants, seeds are the next generation, and serve as the primary means by which individuals of a species are dispersed across the landscape. The grouping of flowers on a plant is called the inflorescence.
In addition to serving as the reproductive organs of flowering plants, flowers have long been admired and used by humans, mainly to beautify their environment but also as a source of food.
http://en.wikipedia.org/wiki/Flower
