(a) Super-close-focus photographs of C@SiO2 beads with four classes of structural colors. Insert scale bar is 1 mm. (b) Schematic diagram of PC beads encoded by structural color (i) and spectrum (ii). (c) Optical photographs of C@SiO2 beads with four classes of structural colors. The insert scale bar is 300 μm. (d) The comparison of the signal area between structural color mode and spectral mode of PC beads.