This article poses and discusses a maximization problem: for a given chromaticity, and in a given illumination, how light a fluorescent color is possible. The article does not present a solution, but does offer a conjecture that for many illuminants and many chromaticities the greatest lightness is achieved by a simple form of bispectral function. The output is in two points plus a tail, so we call it a "PPT" function. (C) 2007 Wiley Periodicals, Inc.