The observed structural colour occurs because of the diffraction
of incident light induced by the silk nanostructured lattice. This can
be quantified theoretically by calculating the bandgap associated
with the lattice geometry and index of refraction21,22 and by
Bragg–Snell diffraction theory (Supplementary equations (1)
and (2)). The SIOs presented here are three-dimensional fcc
photonic crystals with a pPBG in the L-point, [111] crystal direction
of the fcc lattice, which corresponds to the vertical direction of the
SIO film. Owing to the existence of the pPBG in the SIO, vertically
incident light with frequencies falling within the pPBG are reflected
by the SIO. The bandgap frequencies can be controlled by changing
the lattice constant of the opal, either by using PMMA spheres with
different diameters, which changes the centre-to-centre distance of
the voids in the inverse opal, or by changing the index contrast of