PEX tubing comes in many colors, including red, blue, white, orange, black and variations on these themes. Color is not a meaningful criterion in choosing PEX tubing.
Instead, the starting point is the presence the oxygen barrier. PEX with an oxygen barrier is used to be installed in closed-loop heating systems. This type of PEX is red, sometimes blue. PEX without an oxygen barrier is used for potable systems, It comes with the required certifications to be used as a potable installation. Potable PEX is commonly red and blue to facilitate laying out the supply lines. Alternatively, it may be white when differentiating the supply lines is not required. These colors are only to help identifying the supply lines and have no incidence on the property of the tubing.
PEX-Aluminium-PEX is usually orange. PEX used for waste water is usually purple.
Grey is a color reserved for another type of plastic tubing, usually made of polybutylene. Those are not used any longer and have different characteristics than PEX tubing.