PEX tubing normally falls into two main categories: PEX with an oxygen barrier and PEX without an oxygen barrier. PEX with an oxygen barrier is designed for closed-loop space heating systems, and will normally be either red or white. It is usually not approved for use with potable water. PEX without an oxygen barrier is designed primarily for use in potable water plumbing systems. It is available in red, blue, and white to make classification of water lines easier (red for hot, blue for cold, white for supply lines or single-color systems). The color of PEX does not change any of the tubing’s ratings. There would be no problem, for example, using blue PEX for hot water lines or red PEX for cold water lines. Other types of PEX include PEX-Aluminum-PEX, which is often orange, and PEX for reclaimed water, which is usually purple. Gray plastic pipe is often polybutylene — an older type of pipe that is similar to PEX, but that has a different inside diameter and chemical composition.