Your audience will see you blowing a blue balloon, but as soon as you prick it, the balloon will change its colour.
The things you need:
1. Balloons of two colours (blue and pink)
2. A grain of rice or wheat
3. A needle
Step 1: Put the pink balloon inside the blue one and blow into both of them together.
Step 2: Hold out the balloon and prick it with a needle. It is possible that while pricking the balloon, you may land up bursting both of them.
To avoid this, put a grain of rice or wheat between the two balloons before blowing into them.
The blue balloon will burst revealing the pink one. To the others, who do not know the trick, it will seem as though the blue balloon has changed its colour to pink.
You will need a lot of practice to prick the balloon, as well as to conceal the burst balloon before anyone sees it.