The magic box does all the magical work for you.
You can slide out the drawer from the magic box and show your audience the red ball. However this time do not place the drawer back into the casing the same way. Spin the drawer around with the ball resting in the drawer. When you push the drawer back into the casing, the ball will become magically locked in a secret compartment and it will switch colours with another ball.
To change the colour of the ball again, take out the drawer and rotate it. Place it back in. When you remove the drawer again the coin has magically returned.