Widespread concerns over the impact of human activity on the environment has resulted in a desire to replace artificial functional materials with naturally derived alternatives. As such polysaccharides are drawing increasing attention due to offering a renewable, biodegradable, and biocompatible building block for functional nanomaterials. In particular, nanocrystals of cellulose and chitin have emerged as versatile and sustainable building blocks for diverse applications, ranging from mechanical reinforcement to structural coloration. Much of this interest arises from the tendency of these colloidally stable nanoparticles to self-organize in water into a lyotropic cholesteric liquid crystal, which can be readily manipulated in terms of its periodicity, structure, and geometry. Importantly, this helicoidal ordering can be retained into the solid state, offering an accessible route to complex nanostructured films, coatings, and particles. In this review the process of forming iridescent, structurally colored films from suspensions of cellulose nanocrystals (CNCs) is summarized and the mechanisms underlying the chemical and physical phenomena at each stage in the process explored. Analogy is then drawn with chitin nanocrystals (ChNCs), allowing for key differences to be critically assessed and strategies towards structural coloration to be presented. Finally, the progress towards translating this technology from academia to industry is summarized.