Why indigo moves
Indigo is a surface dye that never fully bonds to cotton fibre, which is exactly why jeans fade attractively. That same loose dye transfers to anything it rubs against under warmth and pressure, and a person sitting on a sofa provides both. New or dark-wash denim transfers most. The dye migrates into the leather finish rather than sitting on top of it, which is why wiping does very little and why the discolouration builds so evenly that owners often miss it until it is significant.