positive. The "mandate" is really a "suggestion," and the home team has final say.
I tried looking it up and the league made the switch a few years ago. Haven't found anything definitive but it sure looks like it is mandatory. The other thing I found about it is most of the third jerseys are dark to fall in line with the home team having to wear darker uniforms. The big reason they did the switch was for marketing so they could sell more jerseys. I looked on NHL.com and couldn't find anything on it but may give another look.
I figured it had something to do with the green jerseys they wore in warmups, like CJ said. You'd think if they did it b/c Trotz is superstitious and didn't wanna change anything, he wouldn't have wanted them wearing crazy St. Patrick's Day jerseys during warmups either.
It could have also been something as simple as Philly asking if they could wear darks and Nashville saying OK. Maybe Philly wanted their ex-Preds to feel like they were at "home"
I like the coming off the road trip theory the best. Trotz mentioned in the paper about how tough this game was going to be because of the road trip
The home team is required to wear their dark jersey and the road team their light-colored uniform. However, with enough advance notice (I believe it is six weeks notice), the home team request through the league office that they will be wearing a different color (light or third jerseys).
The advance notice is needed due to the fact that it requires the visiting team to take an extra set of jerseys, socks, helmets, etc to meet the jersey requirement for that night. The league only allows a team to do it occasionally and I don't know the specific reason for last night, but know that it wasn't something decided in the past few days...it requires a lot of advance notice...