The way light bounces off tiny particles or structures in a material, sometimes producing a visible colour without any pigment being present.
Shorter (blue) wavelengths scatter more than longer (red) wavelengths when light passes through a medium containing particles smaller than or comparable to the light's wavelength — a Tyndall- or Rayleigh-type effect. In a low-melanin iris stroma, this scattering effect is what produces the visible blue or grey appearance, not a blue pigment.
Light scattering is a physical/optical effect, not a chemical pigment — it's the same broad family of phenomenon behind why the sky looks blue, applied at the scale of the iris stroma.
Blue and grey eyes get their colour mainly from light scattering in a low-melanin iris, not from a blue or grey pigment.
Explains why 'blue eyes' and 'blue paint' work completely differently, and why blue/grey eye colour is unusually sensitive to lighting and photography compared with brown.