A red sponge ball is displayed openly, both the performer's hands being clearly seen to be otherwise empty. Curling his left hand into a loose fist, the magician pushes the red sponge ball inside and then blows on the hand. Immediately the fist is opened to reveal the ball has changed to yellow and both hands are once again genuinely empty.
This simple effect includes a principle that you will be able to use in many other effects. No gimmicks are required.