Color–color diagram showing the C3 of a blackbody (solid lines) and of SN 1999em (dashed lines) during the photospheric phase. Differences between them are due to the SN not being a perfect blackbody, to the presence of spectral lines, and also to the reddening that affects the SN. Line blanketing makes the difference stronger in the B band. The linear fit is more accurate for the VRI points (σ = 0.02) than for the BVI points (σ = 0.06).