How a disordered material makes color at all
Structural color is color produced by physical structure rather than pigment. In ordered materials like opals, the color comes from Bragg diffraction: light waves reflected off regularly spaced layers reinforce one another at particular wavelengths, and the spacing sets the hue. Photonic glasses are different. They are made of densely packed particles with only short-range order, meaning each particle has neighbors at roughly consistent distances but the arrangement has no long-range pattern. Because the structure is isotropic, the color does not change with viewing angle, which is an appealing property [1].
The same basic architecture shows up in biology. Colloidal glasses, bird feathers, and beetle scales can all display structural colors that arise from short-range spatial correlations between scattering centers, rather than from a periodic lattice [1]. That shared physics is why these three very different systems can be discussed together: the question of missing red is a question about how individual scatterers behave, not about any one organism's biology.
The blue bias of individual particles is what blocks red
The key explanation is that individual particles do not scatter all colors equally. Scattering theory shows that a single particle tends to backscatter light more strongly in the blue than in the red [1]. This bias comes from cavity-like modes inside the particle, resonances that make it a more effective mirror for shorter wavelengths. So even before the particles are assembled into a structure, the building blocks are already tilted toward blue.
That tilt collides with the structural resonance needed for red. To get red from a photonic glass, you need larger particles and larger characteristic spacings, which shifts the structural resonance toward longer wavelengths. But the same particles still backscatter blue strongly, and the two effects interact in a way that prevents a clean red [1]. The practical consequence is visible in colloidal glasses: when particle sizes are chosen to push the structural color into the red, the resulting color is weakly saturated [1]. In other words, red is not simply shifted out of reach; it is diluted.
What beetle scales and feathers actually show
Beetles offer a biological parallel. Beetle scales contain amorphous nanoscale photonic structures, and most beetle scales observed so far lack red structural color [2]. The same source notes that longer-range order has a dramatic effect on structural color, which fits the broader picture that the disordered, short-range-order architecture is where the red problem lives [2].
Bird feathers are more varied. Many feather nanostructures are quasi-ordered rather than strictly periodic, and some birds have evolved more ordered three-dimensional photonic crystals. Blue-winged leafbirds, for example, have bicontinuous single gyroid networks of beta-keratin and air in their feather barbs, structures that evolved from ancestral quasi-ordered channel-type nanostructures [3]. That tells us ordered architectures do occur in feathers, but it does not by itself explain red. The scattering argument in [1] is about the disordered, angle-independent case, so it should be read as an explanation for that class of materials rather than for every feather.
Can red be engineered into these materials?
Yes, at least in principle, and the same theory that explains the gap points toward fixes. The scattering model was used to develop design rules for colloidal glasses that could show red, angle-independent structural colors [1]. That is a design target rather than a demonstrated natural phenomenon, but it means the absence of red is a consequence of how the scatterers behave, not a fundamental prohibition.
Bio-inspired fabrication is already pushing in this direction. One approach mimics pearls and bird feathers with a disordered multilayer architecture that does not require strict periodicity, and it adds broadband-absorbing materials inspired by the melanin in bird feathers to selectively suppress unwanted interference [4]. By tuning those absorbers, the researchers report a continuous structural color spectrum covering the visible range [4]. That is a different structural route from a photonic glass, so it does not overturn the blue-backscattering explanation; it shows that other disordered designs can reach red by managing absorption and interference rather than by fighting the single-particle scattering bias.
Sources used in this answer
Absence of Red Structural Color in Photonic Glasses, Bird Feathers and Certain Beetles
Scattering theory shows that individual particles backscatter blue more strongly than red because of cavity-like modes, and this interacts with structural resonances to prevent angle-independent red color in photonic glasses, bird feathers, and beetle scales, while also yielding design rules for red colloidal glasses.
Photonic crystals in beetles
Beetle scales contain amorphous nanoscale photonic structures, most observed beetle scales lack red structural color, and longer-range order has a dramatic effect on structural color.
Evolution of single gyroid photonic crystals in bird feathers
Blue-winged leafbird feather barbs contain bicontinuous single gyroid beta-keratin and air photonic crystal networks that evolved from ancestral quasi-ordered channel-type nanostructures.
Multi-Biomimetic Structural Colors Inspired by Pearls and Bird Feathers.
A disordered multilayer architecture inspired by pearls and bird feathers, combined with melanin-inspired absorbing materials, can produce a continuous structural color spectrum across the visible range without strict periodicity.
Study of laser actions by bird’s feathers with photonic crystals
Parrot feathers containing photonic crystals can act as scattering materials in random lasers, with higher photonic crystal content lowering the laser threshold.