The medical name for these patches is corns and calluses, and they are usually a result of shoes that are too tight, too loose, or high-heeled. Also, they often appear when wearing second-hand shoes.
Used shoes can easily create toe issues, since they’ve already been shaped to someone else’s foot. Over time, the form of the shoe is altered and pressure points and seams are created, which are not aligned with your own toes. Consequently, this may cause new problems, including friction and pressure where the shoe rubs and the toes are constantly compressed.
In response, the skin thickens to protect itself, forming a hard, raised area that often appears yellow because of a buildup of dead skin cells (keratin). Hard corns usually form on the tops or sides of the toes and may have a central core that feels painful when pressed, often described as feeling like stepping on a small pebble.
Other Possible Causes (Less Common)
Blisters: Prolonged friction may initially result in blisters full of fluid, which may have a yellow appearance if they get infected.