  The ability of mixing colors with remarkable results had long been exclusive
to the talents of master painters. By finely combining colors at different
amounts on the palette intuitively, they obtain smooth gradients with any given
color. Creating such smooth color variations through scattering by the
structural patterning of a surface, as opposed to color pigments, has long
remained a challenge. Here, we borrow from the painter's approach and
demonstrate color mixing generated by an optical metasurface. We propose a
single-layer plasmonic color pixel and a method for nanophotonic structural
color mixing based on the additive RGB color model. The color pixels consist of
plasmonic nanorod arrays that generate vivid primary colors and enable
independent control of color brightness without affecting chromaticity, by
simply varying geometric in-plane parameters. By interleaving different nanorod
arrays, we combine up to three primary colors on a single pixel. Based on this,
two and three color mixing is demonstrated, enabling the continuous coverage of
a plasmonic RGB color gamut and yielding a palette with a virtually unlimited
number of colors. With this multi-resonant color pixel, we show the
photorealistic printing of color and monochrome images at the nanoscale, with
ultra-smooth transitions in color and brightness. Our color mixing approach can
be applied to a broad range of scatterer designs and materials, and has the
potential to be used for multi-wavelength color filters and dynamic
photorealistic displays.
