Many trees have leaves that are green in the summer and red, yellow, or orange in autumn. where were these colors during the summer how can they suddenly appear in autumn

 Inside a leaf there are millions of little "packages" of color in green, yellow and orange.  The green packages are called chlorophyll 
The yellow packages are called xanthophyll 
The orange packages are called carotene 

In the summer, the green chlorophyll packages are very, very busy. The green packages in the leaves catch sunlight and, using it for energy, they change water from the ground and a special gas from the air ("carbon dioxide") into sugar ("glucose"). 

This sugar is the food for the tree. 

All summer long the green chlorophyll produces food for the trees. Because the green chlorophyll is so busy, the green color covers up all of the other colors in the packages. 

As fall approaches, the weather grows colder. The tree realizes that winter is near and begins to get ready. 

A thin layer of cells grows over the water tubes in the leaves and closes them up in preparation for the winter. No more water can get into the leaf! 

Without the water, the green chlorophyll starts to disappear and the other colors in the leaf -- the yellow xanthophyll packages and the orange carotene packages -- can finally be seen. 

The leaves don't really "turn" a certain color -- they just lose their green. 

When fall comes leaves little tubes are covered so they can't let in water. The sap in the trees uses the same tubes to carry the sugar around to feed the rest of the tree. When the wall covers the tubes, sometimes sugar gets trapped inside the leaf. This sugar may cause the sap to turn red or purple. 

And this makes the leaves look red or purple!