The power company where I used to live did something like this for it's online dashboard, but with images (I think it was Delmarva Power). Every time you entered your username to log in, it would show you a simple image/line-drawing and a message saying that if the image ever changes, you're being scammed, so don't enter your password. I've never seen anything like it since.

As far as colours on emails goes, if everyone starts doing it, nobody will remember which colour goes with which company. It needs to be something more distinctive than just a colour band.