Difference between instrumental Gemini magnitudes and the catalog magnitudes of selected stars, vs. catalog color, for the two catalogs used for calibration in Col-OSSOS: SDSS (upper set of panels) and Pan-STARRS1 (lower set of panels). Upper set: SDSS. Top: g band; middle: r band; bottom: z band. Lower set: Pan-STARRS1. Top: g band; bottom: r band. In each case, the derived best-fit linear color term is shown as a red line.