a fruit is green when unrie but become beautifully coloured when ripe.

a) how does this change occur?

b) what is the importance of this change?

(a) ​Fruits and vegetables change colour during ripening process from green to red or yellow or blue due to change in the pigments. Raw fruits which are green due to presence of chlorophyll, change colour during ripening as chlorophyll is broken down and sometimes new pigments are made such as carotenoids. Carotenoids provide various colours to ripened fruits and vegetables. This process is controlled by hormones like ethylene and some enzymes.  

(b) This change (ripening) is important so that the fruits become more less green, more sweeter and softer so that they can be consumed easily.