NOTE: Doing this during a saved game file will delete all the pokemon you ahve in your box!  I found that out the hard way.

Start a new game as either a boy or a girl

Save it right away!

Then start a new game as the opposite sex than you chose last time

And save it again right away

But when it says Saving, dont turn off the power, turn off the gameboy.

When you continue the game, your character will be the wrong color!
Normal Boy:
Wrong Colored Boy:
Normal Girl:
Wrong Colored Girl