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In chemistry, the term chromogen refers to a colourless (or faintly coloured) chemical compound that can be converted by chemical reaction into a compound which can be described as "coloured" (a chromophore).^[1][2] There is no universally agreed definition of the term. Various dictionaries give the following definitions:

• A substance capable of conversion into a pigment or dye.
• Any substance that can become a pigment or coloring matter, a substance in organic fluids that forms colored compounds when oxidized, or a compound, not itself a dye, that can become a dye.
• Any substance, itself without color, giving origin to a coloring matter.

In biochemistry the term has a rather different meaning. The following are found in various dictionaries.

• A precursor of a biochemical pigment
• A pigment-producing microorganism
• Any of certain bacteria that produce a pigment
• A strongly pigmented or pigment-generating organelle, organ, or microorganism.^{[citation needed]}

• In chromogenic photography, film or paper contains one or many layers of silver halide (AgX) emulsion, along with dye couplers that, in combination with processing chemistry, form visible dyes.

• The Runyon classification classifies mycobacteria by chromogenic properties.