Dispersions of graphene oxide nanoflakes in water can be manipulated into photonic crystals that reflect the full spectrum of visible colour. Normally, dissolving nanometre-thin graphene oxide crystals in water produces brownish solutions. Jang-Kun Song from Sungkyunkwan University in South Korea and co-workers used extensive oxidation and cleaning procedures in combination with bottom-up and top-down control strategies to coax unexpected optical activity from the irregularly shaped nanoflakes. The team's synthetic procedure produced a strong electric double layer on graphene oxide surfaces that induced the particles to stack into periodic, light-reflecting structures similar to cholesteric liquid crystals. Adding salt ions to the watery dispersion allowed the photonic structures, and hence the colour reflections, to be adjusted at molecular scales. Alternatively, the researchers could ‘paint’ glass slides by using shear forces produced by stirring to direct colour output.