A leather patina is a sheen on the surface of the leather that forms through use and exposure to the sun which provides water resistance, character, & personality to the product.
The oils in your hands & the light from the sun are some ways that your wallet will darken & develop a patina.
If you desire faster results, you can apply a leather balm more frequently. This will enrich the leather with oils & wax, creating a worn-in effect.