Question

why gas cylinder is always red

Answer 1

Red colour is a sign of danger and it has always been used for dangerous objects or hot objects. LPG cylinders being inflammable and thus dangerous, are painted red in colour , specifically the signal red colour.
 
Scientifically, Red colour has the highest wavelength in the visible spectrum which makes it least scattered by air molecules. So, Red is able to travel to longest distances and thus it is the easiest colour to detect from a distance. That's why Red colour is used for dangerous objects. 

Answer 2

Red colour is a sign of danger and it has always been used for dangerous objects or hot objects.so the gas cylinder is red in colour