Metamerism is an effect that's usually (in photography) associated with viewing prints, especially ink jet prints. Dyes inks, and to a lesser extent pigment inks can appear to be color shifted depending on the type of light they are viewed under. This effect varies with the ink formulation. I know of no way to control or minimize this at the 'taking' end of the process.
One can compensate for metamerism by viewing a print under the light it's likely to be seen and correcting the colors accordingly. It's possible that photographing a gray card at different angles gives a neutral-light and a neutral-dark that allows easier compensation.