Patina is a color with no exact numerics to be mimicked. Instead, it's a color achieved through a process. According to Oxford Dictionary, it is a 'green or brown film on the surface of bronze or similar metals, produced by oxidation over a long period'.
The name comes from the Latin word 'patina', meaning ‘shallow dish’. The color is produced through a chemical process called patination, in which a work of art or surface made out of copper, bronze, or metals. When a metallic surface such as a bronze one is coated by a patina, a thin layer made for protecting things from age and wear, it will turn to a certain green. This is due to the exposure of chlorides.
Take notice of the Statue of Liberty. Its copper surface has a greenish color thanks to the natural patina that covers it. The buildings of the Nordic Embassies in Berlin are also covered in natural patina, giving that light, bluish green. The brass statue of Joseph I of Portugal in Commerce Square, Lisbon was also patinated, originally having the same light green until the removal of its patina in 2012. While patina is more associated to this green shade, it can take in various colors such as blue-black with ammonium sulfide, brown-black with liver of sulfur, or yellow-brown with ferric nitrate.
While this color does not come naturally, it's pretty easy to capture it in photography. Just use blue and green filters for your camera and cross-processing with Lomography Color Negative 400.