This is powerful little piece of mental magic. You will need 3 small slips of paper, a pen and 3 markers (a green, red and yellow.) With the pen, draw a simple traffic light on each of the three pieces of paper. Each of the drawings should feature three circles in a single column, inside a rectangle.
In one of the drawings, color in one of the lights at one end of the rectangle GREEN. In another drawing, color in one of the lights at the other end of the rectangle RED. And in the third drawing, color in the light in the middle of the rectangle YELLOW.
Crumple up the three slips of paper into small balls. Put the GREEN drawing in your shirt pocket, the RED drawing in your left pants pocket, and the YELLOW drawing in your right pants pocket.
To perform, remove the crumpled GREEN paper from your shirt pocket and place it on the table or give it to someone to hold. Then casually slip both hands into their respective pockets and adopt a rest position.
Ask someone to imagine "you are driving a car along a road and you come up to a street light. What color is the light? Green, yellow or red?" I have found that at least 50% of the time people say 'green' and then I ask them to open the crumpled piece of paper to discover the green light.
As they do, I casually slip my hands out of my pockets with the balled up RED drawing hidden in my left hand. (See 'Notes' for what I do when they DON'T say "Green.")
After everyone has had a chance to see the drawing with the GREEN light, take it back with your empty right hand and crumple it again into a small ball. Pretend to transfer the ball into your left hand, but actually retain the GREEN drawing in your right hand and switch-in the RED drawing. Casually deposit the RED drawing on the table or hand it to someone on your left as you slip your right hand back into your pocket for a deck of cards, coins or whatever in preparation for your next effect.
Begin the next effect, but stop yourself after only a few moments and gesture to the crumpled drawing as you say, "Oh, and just in case you are wondering what would've happened if you had said red...." Snap your fingers over the drawing and go right back to the first few words of your next effect.
Very soon someone will pick up the paper ball and unfold it to find the light has mysteriously changed to RED. Act nonplussed without being arrogant, as if such extraordinary events are part of your everyday reality (which they hopefully ARE.)
NOTES: The other 50% of the time, when people don't say the light they imagined was GREEN, I immediately palm the named color drawing in either my left or right hand and slide my hands out of their respective pockets at the same time. With my empty hand I pick up the crumpled GREEN drawing and pretend to transfer it to the other hand, but of course switch-in the correct colored light. In a continuing action I hand the switched-in paper ball to someone to open and continue as already described.