Why streetlights on Milwaukee interstates, highways are purple

Do you know why some Milwaukee highway and interstate lights are purple?

Years Ago, You Could Buy A Ford F-Series With A Manual Transmission, One Of The Other Greatest Truck Engines, And Get It In Purple: Holy Grails - The Autopian

Streetlights Turn Purple in Milwaukee – Exponent

WisDOT: Manufacturer defect causes purple-blue lights on highway

Have y'all noticed they are replacing white street lights with purple colored lights? : r/Pensacola

I noticed purple street lights near my place in northern Maryland and I thought maybe those were defective. Then this is what I saw when my flight was landing at BWI tonight.

Why the purple street lights? : r/Austin

1,646 Wisconsin Highway Images, Stock Photos, 3D objects, & Vectors

Why some street lights on I-93 — and around Boston — are bright purple - The Boston Globe

Only a handful of purple streetlights remain on Wisconsin interstates