Illustration of the flux ratio measurement. The upper panel shows the input spectrum (de-redshifted and corrected for Galactic reddening; here SN 1999gd around maximum light), binned on a log-wavelength scale. The gray vertical lines represent the fluxes in characteristic wavelength bins mentioned throughout the text. The lower panel shows the same spectrum corrected for SALT2 color, which is used to measure the color-corrected flux ratios ℛ^c.