A new method to control the iridescence color of solid nanocrystalline cellulose (NCC) films by ultrasound and high-shear (mechanical) energy input to the NCC suspension prior to film formation is provided. As the energy input to the NCC suspension increases, the resulting film color shifts from the ultraviolet region towards the infrared region of the electromagnetic spectrum; this wavelength shift lies in the opposite direction to that caused by the addition of electrolytes to NCC suspensions prior to film formation. No additives are required to achieve the changes in color; color changes can also be effected by mixing two suspensions exposed to different levels of sonication.