Apparently it's an easteregg/hidden feature, but it's pretty useful for placing pipelines that aren't supposed to ever hook up and for telling them apart visually, too.
Conveyors being colorizable is in the manual, too. That one got added later, so maybe this one just fell off the radar?

Assigning this to myself, since it's a gameplay/ingame documentation topic, and also not a bad thing to put in 1.16 as well.