Chad Graff on Twitter: "The Vikings will debut a new helmet color tomorrow. The old matte purple helmet (since 2013) didn't always look like it matched the purple jerseys in TV broadcasts.
Heritage Uniforms and Jerseys and Stadiums - NFL, MLB, NHL, NBA, NCAA, US Colleges: Minnesota Vikings Uniform and Team History
Minnesota Vikings throwback uniforms are on their way back
Bears to wear throwback jerseys for Week 4; will Vikings follow suit? - Daily Norseman