Overview The blue-green color of Pseudomonas on agar is due to pyocyanin Pseudomonas aeruginosa blue-green pigment - pyocyanin ubiquitous in "blue-green water" Actinomyces israelii yellow "sulfur" granules "the yellow, sulfurous sand of the Israeli desert" Staphylococcus aureus yellow pigment "aureus (latin) = gold" Serratia marcescens red pigment "Mars is red"