Plant colour

Plant colours are very noticeable, especially in flowering plants, and especially the colour green. Plants are green because it is a functional colour. They photosynthesise, and the chlorophyll they use happens to be green. However, flowering plants show many other colours. Blue is quite rare, though bluebells are a famous exception. Apparently the colour is difficult to make by the usual plant biochemistry. It is also rare in animals, so if it is difficult for plants to make, presumably it is difficult for animals to make.
Colour can be made in two ways. There are pigments which show a particular colour, and there are structural colours.
Plant pigments make the plants show colours. The main pigments are:
There are also structural colours, although they are less common. They are caused by reflecting iridescent colour from a multi-layered surface. Butterfly wings are a typical example.
Some particular types of animals and plants have evolved colour systems which are unique. 
In the course of evolution, many other colourful molecules have been used by plants and animals. In bacteria, dinoflagellates and fungi, for instance, many other colour-causing compounds have been used. Sometimes the colour is just incidental, and sometimes it has a function on the life of the organism.
At first, no animals existed which could see colours. At that stage, colours on plants (etc) were incidental by-products of their physiology. The evolution of colour vision undoubtedly had an effect on the colours of plants and animals, but we have little evidence of this early stage. We know from the plants and animals of today that colour vision is very important.