SATBLU2 had a Fe/Co value of approximately 10 (table 2), and the SATBLU samples had the most saturated color (table 1). GREBLUE2, with a Fe/Co ratio of about 425 (table 2), had a visible gray hue component (table 1). For intermediate Fe/Co ratios of about 130 (measured on SKYBLU1; see table 2), the spinel had a sky-blue color (table 1). Moreover, sample GREBLU1, which had the grayest color, showed more significant bands due to Fe2+. We propose that the GREBLU samples are colored mainly by iron and the SATBLU samples by cobalt. The SKYBLU samples’ colors arise from both iron and cobalt absorption. Hue differences are more significantly controlled by iron (with different species), while saturation is largely dictated by cobalt (D’Ippolito et al., 2015).