2. an area of discoloration of the skin. acid-fast stain a staining procedure for demonstrating acid-fast microorganisms. differential stain one that facilitates differentiation of various elements in a specimen. endogenous stain an intrinsic stain acquired during tooth development.exogenous stain an intrinsic stain acquired after a tooth has erupted.. an area of discoloration of the skin. acid stain. a stain in which the coloring agent is in the acid radical. basic stain. a stain in which the coloring agent is in the basic radical.