In Mycobacterium staining, the organisms are lightly stained. Which of the following is a cause?

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In Ziehl-Neelsen acid-fast staining, the dye carbol fuchsin is driven into the waxy mycobacterial cell wall by heat, and then an acid-alcohol decolorizer is used to wash away the dye from non–acid-fast cells. Mycobacteria resist decolorization, so they stay red, while many other organisms become colorless. If the decolorizing step is left on for too long, even the acid-fast organisms can lose their stain, producing pale or light-stained bacilli. That’s why leaving acid alcohol on too long leads to light staining of the organisms.

Context: heating aids dye penetration, so not heating can yield faint staining as well, but the specific issue of light staining described here is best explained by over-decolorization. Short or insufficient decolorization would tend to leave cells red; staining time itself mainly affects intensity of the dye, not the decolorization outcome.