I was watching a U.S. college bowl between Mississippi St. and UCF and both teams were wearing their dark uniforms. I thought it looked great! And then I thought, gee, I wish the CFL would do that, even if it was just in Grey Cup games.
Most Eastern teams don't have the same dark colours as Western teams, so why not? In theory you could have Hamilton in black vs Calgary in red, or BC in orange vs Toronto in blue etc. Obviously if there were two teams with the same colours playing each other like Winnipeg vs Toronto (both blue) you'd go with one team in white. But generally I find the white uniforms boring (I'd rather see the 3rd jerseys used than the whites) and I really like the coloured ones. It's just an idea and I don't really see any downside, unless you want to say it goes against tradition.