Butterfly wings come in almost any color imaginable. In order to make these colors, they use a combination of structural color and pigments. Pigments are molecules that absorb or reflect certain colors of light, like the pigments in paint, whereas structural color is determined by the shape of nanostructures and the materials they are made of. A great example of this is the green forester moth, which can change its color by trapping moisture in the nanostructures on its wings. 

A new study explored how buckeyes, butterflies from the genus Junonia, created their color, and how this trait was controlled by their genes. Scales on the wings of butterflies are made of chitin, and generally consist of two structures: ridges on top and a thin film below. The thickness of this film plays a role in the color the scale will have. 

For their study the scientists used a special type of buckeye: one that was bred to be blue. Normally these butterflies are for the most part brown, but will have small patches of blue. Edith Smith, a professional butterfly breeder, selected the individuals with the largest patches and crossed them, until she eventually got butterflies that were for the most part blue instead of brown. 

The main difference the researchers found between these special blue buckeyes and the normal brown ones, was the thickness of the film, also called the lamina, in their scales. By selecting the blue butterflies, the breeder had actually selected the butterflies with the thickest films, which was responsible for the color. They researchers also found that they could mimic this effect by deleting the optix gene, which is important for the color of butterfly wings.

These results are not just true for one species of buckeye. The researchers found this link between lamina thickness and color 10 different species of butterfly. Sadly, the lamina cannot make every color in the rainbow. So butterflies cannot just rely on their lamina, they also need to use pigments and other clever tricks to create their brilliant colors.