Tubing, casing and coupling should be sprayed with color code to distinguish different grades of steel. The color code of tubing pipe and casing pipe is to spray the color ring at the distance of more than 600 mm from any end. The coupling is sprayed with color on the whole outer surface, and then the color ring is sprayed.
The color code of tubing, casing and collar are shown in the table below.
Basically, you can know what grade the tubing and casing are from the color code.