why is Pseudomonas aeruginosa green in colour when cultivated?

Answer 1

It depends in which medium it is grown.

Pseudomonas aeruginosa (also known as Pseudomonas pyocyanea) is a Gram-negative, aerobic, rod-shaped bacterium with unipolar motility. Like other Pseudomonads, P. aeruginosa secretes a variety of pigments, including pyocyanin (blue-green), fluorescein (yellow-green and fluorescent), and pyorubin (red-brown). King, Ward, and Raney developed Pseudomonas Agar P (aka King A media) for enhancing pyocyanin and pyorubin production and Pseudomonas Agar F (aka King B media) for enhancing fluorescein production.
Definitive clinical identification of P. aeruginosa often includes identifying the production of both pyocyanin and fluorescein as well as its ability to grow at 42°C.

Answer 2

I'm not sure what the structure of the pigment is, but I would guess that you could easily google "pseudomonas aeruginosa pigment structure" to find out....

Basically, the cells produce this green pigment...