  Varying only the in-plane or out-of-plane dimensions of nanostructures
produces a wide range of colourful elements in metasurfaces and thin films.
However, achieving shades of grey and control of colour saturation remains
challenging. Here, we introduce a hybrid approach to colour generation based on
the tuning of nanostructure geometry in all three dimensions. Through
two-photon polymerization lithography, we systematically investigated colour
generation from the simple single nanopillar geometry made of
low-refractive-index material; realizing grayscale and full colour palettes
with control of hue, saturation, brightness through tuning of height, diameter,
and periodicity of nanopillars. Arbitrary colourful and grayscale images were
painted by mapping desired prints to precisely controllable parameters during
3D printing. We extend our understanding of the scattering properties of the
low-refractive-index nanopillar to demonstrate grayscale inversion and colour
desaturation, with steganography at the level of single nanopillars.
