Practicing colors. The main color is most saturated, both the highlight and shadows are of lower saturation and with random color shifts. I think it's because of the reflectivity of the materials, when there are too much or too little light bouncing off a surface (as in highlights/shadows), the saturation drops.
Highly reflective materials (skin, metals) has large areas of highlights, so they generally look less saturated overall.
Based on ティーナ・ディアボラ from さて、異世界を攻略しようか。