Ref had a video of him doing this with Ganondorf, but it's no longer on YouTube. So, here's how to do it, and I'm going to use Ness in this example. Let's say you want to be the Bumble Bee color, and you want the CPU to also be a Bumble Bee color. This also works for other human players, and you can do this for all four characters.
So, you select Ness for player 1 and 2. Now, what you do is select the Bumble Bee color for player 1. You can start off with player 2 first, but in this example, we're going to use player 1 first, and he's going to be starting off with the Bumble Bee color. Once you do that, do the following.
Now that you have player 1 turned off, make player 2 the Bumble Bee color. Once you do that, do the following.
Where the arrow is pointing at, click on that. You'll be doing Team Battle once you do this. After that, turn player 1 back on. Once you do that, you should have player 1 and 2 open. Then click on that same spot where it shows the arrow and both of you will be the Bumble Bee color.