Calf or calf hide is a common form of leather binding. Calf binding is naturally a light brown but there are ways to treat the skin to create different decorative looks.

Calf skin bindings include:

Diced Calf: Decorative diamonds or squares cut or scored into the leather.
Marbled: The leather is stained with diluted acid to produce a swirling effect.
Mottled Calf: The diluted acid from marbling is applied here to produce splotched designs.
Paneled Calf: A rectangular design of framed lines tooled into the leather or expressed with gilt on a cover or spine.
Polished Calf: A reflective finish created by polishing the leather.
Reversed Calf: The inner side of the calf skin is facing outward.
Spanish Calf: Flecks dyed into the calf skin with red and green acid.
Speckled Calf: A diluted acid mixture that creates small dark specks, or spots, on the leather
Tree Calf: Stained by chemicals to produce a dark pattern on the boards.

an example of a book bound in calf