Yup, the original paper (https://www.nature.com/articles/s41598-018-31890-9.pdf) seems to suggest this: “SRAF image composed of merged three pseudo-colored red, green and blue (RGB) channels. Brightness and contrast of the RGB channel intensities are adjusted to highlight the dark crystalline fabrics.“

> They identified organic matter and calcium crystals with ultraviolet light, which uses different wavelengths to make distinct minerals glow.

> A high-resolution method, called Airyscan super-resolution microscopy, captured colorful snapshots of organic matter and crystal layers in the kidney stones, “crosscut and truncated” by newer crevices, triangles and other geometrics, Dr. Fouke said.