Streptomyces rishiriensis is a Gram positive bacterium that…

Streptоmyces rishiriensis is а Grаm pоsitive bаcterium that prоduces dark melanin pigments and a number of antimicrobial compounds. You hope to find gene(s) responsible for the production of these melanin pigments. You transform a culture with a plasmid containing a transposon and a kanamycin resistance gene (kanR) and successfully isolate transformed mutants. You grow them in culture and spread them on the surface of multiple TSA plates. Following incubation, you observe mostly dark colonies along with an occasional clearish/white colony. What might best explain the presence of these clearish/white colonies?