Why does pitcher-plant feeds on insects though it is green?

The green colour in plants is due to the presence of chlorophyll which is used to combine water and Carbon dioxide to form sugars, plants use these sugars to supply energy but plants need many trace elements such as magnesium, manganese, potassium etc to make complex plant alkaloids and organic compounds.

They usually obtain these minerals directly from the soil but most insectivorous plants grow in waterlogged soil from which the nutrients have been leached away by the water. They have therefore evolved an alternative way of gaining the nutrients that they require.