  Spectral rendering is essential for the production of physically-plausible
synthetic images, but requires to introduce several changes in the content
generation pipeline. In particular, the authoring of spectral material
properties (e.g., albedo maps, indices of refraction, transmittance
coefficients) raises new problems.While a large panel of computer graphics
methods exists to upsample a RGB color to a spectrum, they all provide a
one-to-one mapping. This limits the ability to control interesting color
changes such as the Usambara effect or metameric spectra. In this work, we
introduce a one-to-many mapping in which we show how we can explore the set of
all spectra reproducing a given input color. We apply this method to different
colour changing effects such as vathochromism -- the change of color with
depth, and metamerism.
