Usually you can assign materials for pipes using object styles in the Manage tab. This will apply a common material for all pipes.Just like the image below.
Lately I have been working on a project that required me to present a rendered view, clearly showing different pipes with different materials.Say PVC pipes with PVC materials and Copper pipes Showing Copper materials.The Key to the whole plan is the GROUP tool.
Create the pipe runs, complete the various service thouroughly with the systems.Once it reaches a fair stage group the pipe runs separately. Like image shown beside.This may look a lot tedious but this is better than nothing I guess.
In the project Browser under groups select the pipe groups and use the save group tool and save as a separate RVT file .
ome back to the original file and the select an individual group, say PVC like in the image below.
the following prompt appears and choose the command that says to replace with existing file.