And it is green. Its main functions are photosynthesis

Stipules
A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole resembling a small leaf. Stipules may be shed or not shed.

Poinsettia bracts are leaves which have evolved red pigmentation in order to attract insects and birds to the central flowers, an adaptive function normally served by petals (which are themselves leaves highly modified by evolution).
In the course of evolution, many species have leaves which are adapted to other functions.

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  Thorns help protect the plant from being eaten.
  
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  Vines help the plant to attach to surfaces, and helps to climb trees.
  
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  Some leaves are used to store energy in bulbs. An example is the onion.
  
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  Many succulents store water in some of their leaves.
  
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  Some plants (called epiphytes) grow on other plants. They do not have roots in the ground. Their capture rainwater.
  
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  Carnivorous plants use adapted leaves to capture their prey.
  
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  Sliced leaves reduce wind resistance.
  
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  Hairs on the leaf surface trap humidity in dry climates.
  
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  Waxy leaf surfaces reduce water loss.
  
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  Large surface area provides area for sunlight and shade for plant to minimize heating and reduce water loss.
  
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  In more or less opaque or buried in the soil leaves, translucent windows let the light in.
  
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  Succulent leaves store water and organic acids.
  
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  Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g. eucalyptus).