Colonies of Mycobacterium tuberculosis on Löwenstein-Jensen medium after 6 weeks of cultivation, 37°C. Growth of most mycobacteria is slow compared with that of common bacterial species. Mycobacterium tuberculosis belongs to the group of slowly-growing (on solid media it forms colonies clearly visible to the naked eye in more than 7 days), nonpigmented mycobacteria (non-chromogenes). Colonies are pale yellow or buff and color does not intensify after light exposure. More about pigmentation of mycobacteria you can find at Wikipedia