"Reddener" is an interesting word that refers to something that makes someone or something become red. There are a variety of synonyms that can be used in place of "reddener" depending on the context. For example, if you are referring to a blush on someone's face, you might use synonyms such as "blusher" or "flusher." Other possible synonyms include "stainer," "rouge," and "dye." In some cases, you might also use words like "darkener" or "intensifier" to refer to something that makes a color appear more vivid or saturated. Ultimately, the choice of synonym will depend on the specific meaning you are trying to convey.