Why do flowers have different colours?

Flowers first evolved over 2,000,000 years ago. They needed to attract the plentiful supply of insects to help in pollination. The more attractive a flower could present itself, the chances of it passing on its genes increased. All the colours we see around us are reflected colours in light waves, just like the rainbow. That means the red rose you are looking at reflects all other colours except red. It absorbs the red wave length.

to attract certain insects and birds who will recognize what it is by virtue of its color.

Different pigments.

to attract different insects?? Isn't that why flowers have pretty petals in the first place?

Most flowers have adapted their shape and color to enhance pollination. For example, red flowers produce nectar and the combination of the red color and the nectar attracts hummingbirds. The hummingbirds drink the nectar, and then they spread the pollen to the next flower. Plants that don't rely on animals for pollination will not have showy, colorful flowers. Some flowers are certain colors because humans have bred them into those colors just to be pretty.

Each plant (as well as animals) rely on their genetic code to create their characteristics. The genetic makeup, DNA, potentially permits selected colours. This DNA is likely to have mutated over the years and produced a selection of characteristics, including colour ranges. Evolution would be the broadening of the characteristics of species, including the more likely survival of a trait that had advantages over others. Flowers may be pollinated by insects, birds and wind. Those plants pollinated by wind have no real need for flower colour. With the others, attractive patterns, flower shape, colour etc would potentially offer a bigger chance of reproducing seeds and possible domination over different flowers of the same plant type. Many flowers also have highly distinctive shapes and colours, aimed to attract specific animals to pollinate them. The animal may get nectar for food as a win-win deal. The bee orchid's flower (Ophrys apifera) looks just like a bee. In part of its coverage it is surprisingly only pollinated by a bee! During man's domination of the world, it is also thought likely that the prettier flowers would have been saved, at the expense of uglier 'weeds'. This means we have influenced flower colour over time also. See 2.

That is a good question, thinking of it logically, ide say it would be to attract the differant insects or butterfly to pollinate the flowers.

because they like to show off