[Iwonda (OP)]

Just a bit curious here, what are the smallest structured known to science that can display bold beautiful colors?

[Kryptid]

I'm guessing (possibly naively?) that it's on a similar size level to the smallest wavelength of visible light, which is around 400 nanometers, if I recall correctly.

[Bored chemist]

In principle, a single atom can be coloured.
A layer of ink thinner than the wavelength of light would usually be visible.