Blue feathers, blue water, blue flowers, blue butterflies …

The reasons why our eyes perceive these things as blue (or don’t, if we’re colour-blind) vary depending on what we’re looking at. With birds’ feathers, for example, the blue is actually a ‘structural colour’, because ‘When white light strikes a blue feather, the keratin pattern causes red and yellow wavelengths to cancel each other out, while blue wavelengths of light reinforce and amplify one another and reflect back to the beholder’s eye’ (from the amazing Smithsonian website).

Water looks blue because it more readily absorbs light in red, orange and yellow wavelengths but reflects the blue wavelength, so it’s reflecting the colour of the sky above. Blue flowers occur in plants that are able to chemically modify red anthocyanin pigments to reflect white light as blue, and those butterflies that look blue are also reflecting white light, the blue colour being dependent on the placement of the minute scales on their wings. Colour is so complicated!