At home depot they have red and blue copper piping the blue cost twice as much as the red. The only visual difference I can see between the two is the blue looks more polished
Red markings are Type M copper. It is the thinnest walled copper pipe. Blue is Type L copper. It is thicker and used in most house plumbing. There is also Orange marked pipe which is Type K and is normally used for water service lines underground and is the thickest copper.
One is used for water service which is a higher pressure 60-100 and the other is used for lower pressure applications like baseboard heating which is 15-30 based on the previous answers the blue is the higher ores sure pipe and the red is the lower pressure pipe
