This is a picture of bones, the mineral density of bones, giving us a hint of their interior structures.
From the NIH Image Gallery description:
This image shows micro computed tomograophy (micro-CT) reconstructions of femurs of 9-week-old male of Dlx3+/+ (left half) and Dlx3Prx1-cKO (right half) mice. Images are mid-bone sagittal sections with gradient colors corresponding to bone mineral density.
Credit: National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), Laboratory of Skin Biology, Maria I. Morasso, Ph.D., Chief
National Institutes of Health