"I can tell the color of a crayon just by feeling it!" is the opening line to this simple (yet convincing) trick. And remember, a magician never reveals her secrets.
To pull it off, hold both of your hands behind your back and have a skeptic place a crayon in them. (Any crayon will do; just be sure to emphasize to your audience that while you may not get the exact name on the label, you will guess the right hue.)
Then, keeping the crayon behind your back, raise one hand to your forehead and announce the color to your audience.
The Secret: When the crayon is placed in your hands, secretly scrape a bit of wax from one end with your thumbnail. Make sure that is the hand you raise to your forehead so you can subtly take a peek at the wax shavings under your thumbnail to figure out the crayon's color.