Pus is an end-result of suppuration. It may vary in color: yellowish, greenish, or brownish hue. The color may depend on the causative agent of the infection.
Suppuration is the process of forming a pus in the inflamed tissue. However, suppuration may also refer to the pus itself. Pus forms when infective microorganisms (e.g. pyogenic bacteria) enter and invade the tissue and this leads to series of events that trigger the immune action of leukocytes. Chemical messengers are released into the bloodstream, which activate leukocytes to move into the site of infection. Pus is in fact the result of the immune response of the body through leukocytes attempting to remove the infective microorganisms and the affected cells of an injured tissue.