All (most) plants start green because they have something called chlorophyll in them... which helps turn sunlight into energy.
When there starts to be other chemicals present, the color changes as when a fruit is growing or when a leaf gets old.
Find out a bit more at:
http://www.pa.msu.edu/sciencet/ask_st/081496.html
Answered Dec 30, 2010
Edited Dec 30, 2010