The comet is green due to the presence of diatomic carbon (C2). 

Sunlight energizes the molecule and makes it glow.

Photographer is Gerald Rhemann on Dec. 23, 2014, in Namibia.

Good explanation:

Comets are essentially piles of rock, gravel, and dust held together by various kinds of ice (in general, water and carbon dioxide ice). As they near the Sun, the ice turns into a gas and blows away due to pressure from the solar wind and sunlight itself. This also releases a lot of dust and gravelly bits, which tend to follow the comet around the Sun—if the Earth’s orbit intersects this material, we get annual meteor showers.