a Metropolitan Council newsletter article from last month which laid out some of the new names.
The Central Corridor will become the eastern half of the Green Line, while the Southwest Corridor will eventually become the western half. Buses that run on I-35W to BRT stations like the one at 46th Street station will go under the Orange Line moniker, while MN-77/Cedar Avenue BRT will be known as the Red Line. The Northstar Line is not getting a color—appropriate since it doesn't have sufficient service frequency (though apparently that won't prevent them from including it on their maps anyway).
The council is also looking to create a brand name for the entire BRT/LRT system. They got set up to receive suggestions starting back in December last year, though the suggestion period ended on March 18th. (I'm sure many people suggested "Twin City Rapid Transit", just like I did.)
I'm not a big fan of colors for route names, but I suppose I'll adjust. Mostly I just like the "Hiawatha" name. However, I suppose colors would be an improvement over the names "Central"/"University" and "Southwest", which would probably get used if colors didn't exist (although I have a bit of a soft spot for calling the Central Corridor the "Interurban", a common name used for the old TCRT "St. Paul–Minneapolis" line). Either way, they'll be annoying to disambiguate on Wikipedia and harder to set up Google News alerts for...