When you change your Steam profile picture to one where the dominant color is blue.

Go medic and lead the team, the autobalance will do the rest.

Do you mean this?

...or do you mean this?

View attached image.

That's racist, red and blue ppl will be like:

┬─┬ ︵ /(.□. ）

Now.

When you stop defending and attack in team fortress 2 than you change from red to blue

Just join the blue team. Not so hard. Choose it on team selection.

Or, read the section on Background Colors