The Manual on Uniform Traffic Control Devices is one of the least boring and most understandable manuals for the application of engineering in the real world. It standardizes all signs and lane striping used on U.S. roadways, from local streets to multi-lane freeways. Most countries have adopted the MUTCD as the basis for their highways standard.
It is the MUTCD that dictates that all signs giving directions are green with white letters, signs for historical and cultural sites and National Parks are brown with white letters, etc. In its most recent edition the MUTCD introduced a brand new color, purple for the exclusive use of controlled express lanes such as High Occupancy Toll lanes and lanes with electronic tolls.
There is a funny side to this purple color designation for express and HOT lanes. Agenda-controlling Kapolei Neighborhood Board Chair and unsuccessful Hawaii Legislature candidate Maeda Timson is a very vocal pro-rail advocate; she is the current president of Go Rail Go. Her signature color is purple. It is all but impossible for Maeda to appear in public with not at least one purple article of clothing.
Ironically, purple HOT or Express lanes is the only real traffic congestion solution for Kapolei. Maeda can paint the train purple but, as the city has published widely, in 2030, congestion for Kapolei commuters will far worse with a purple train, than it is now without it. Given that the train is stuck in environmental and fiscal problems, there is plenty of time for the Go-Rail-Go president to switch to the correct transportation solution with her most preferred color coming standard and not as special order.