Final answer:
Certain bacteria, such as Pseudomonas aeruginosa and green sulfur bacteria, produce pigments that give bacterial sputum its green color.
Step-by-step explanation:
The green color of bacterial sputum is caused by the production of pigments by certain bacteria, such as Pseudomonas aeruginosa. These bacteria produce a green pigment called pyocyanin, which gives the sputum its green color. Another example is the green sulfur bacteria, which produce green bacteriochlorophyll.