Sometimes the skin on your baby's arms and legs will develop a pattern that looks sort of like marble. This occurs when the blood flow to the skin diminishes. It is a difficult condition to photograph.
Mottling happens to all of us if we get cold. Our sympathetic nervous system shunts the blood away from our skin so we don't loose body heat.
Babies get mottling because their sympathetic nervous system isn't regulated yet. If your baby is otherwise acting healthy, it should go away in a few minutes.
Thanks to Janelle Aby MD, Stanford School Of Medicine, Newborn Nursery, and Lucille Packard Children's Hospital for the use of occasional photographs.