As far as I am aware, the sole purpose of coloured petals is to attract pollinators. Obviously, there are two main types of pollination; wind and insect, and the flowers of plants in these two catagories have very different characteristics. For example, wind pollinated plants have large feathery stamen hanging outwith the petals (if any) to catch pollen of other plants of the same species that are floating on the wind. They have dull petals as they don't have to attract insects.
Perhaps, because of the often humid and windless atmosphere of equatorial rainforests, wind pollinated plants are few and far between? There is also the fact that these 'insect pollinated' plants need to attract a wide range of - not only insects - but birds, mammals and reptiles..
