Runyon I organisms (photochromogens) are slow growing, and produce a yellow-orange pigment when exposed to light.
In contrast, Runyon classifications III through IV are considered nonphotochromogens, in that exposure to light does not make them produce pigment which they would not develop in dark growing conditions.
[3] Runyon II organisms (scotochromogens) are slow-growing and produce a yellow-orange pigment regardless of whether they are grown in the dark or the light.
Runyon III organisms (nonchromogens) are slow-growing and never produce pigment, regardless of culture conditions.
Runyon IV organisms are rapid growing for mycobacteria (colonies in 5 days).