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What colour makes tree frogs so vibrantly green? The blue beneath their skin, of course. Those are the findings of a new study, which reveal that a unique protein complex that reflects blue light is responsible for an unusual green—helping them blend into their surroundings and evade predators.

Scientists came across the complex while trying to understand how hundreds of tree frog species can accumulate large amounts of a toxic green pigment known as biliverdin. In most animals, biliverdin is so dangerous that it is immediately broken down or excreted. In humans, it forms when red blood cells break down and cause the greenish colour sometimes seen in bruises. But in these frogs, it builds up to what should be intolerable levels.

When the researchers isolated the pigment in eight species, they found it stayed stable—and innocuous—by binding with another protein called serpin. The biliverdin-serpin complex was found throughout the body, in the lymph, muscles, and skin. And because frog skin is mostly yellow, it looks bright green wherever the protein is present.