API Casing Color Code
To determine the strength of various casing materials, API has designated defined grades. This grading system consists of a letter and number as P-110, L-80. The number is an arbitrary one (e.g. K-55) representing the min yield strength of the casing (Casing performance Properties), the number is an arbitrary one (e.g. N-80). There are also non-API casings that we use in special cases as deep wells or in high-corrosive environments.
https://www.drillingmanual.com/api-casing-color-code/