STEP 1: Go to Walgreens, Wal-Mart, Meijer or some other local temporary hair color vendor and purchase one can of Jerome Russell B-Wild Temporary Color Spray in Jaguar Green*.Hours of fun guaranteed. Enjoy.
STEP 2: Follow directions on can to coat hair with color spray. Ignore line of directions that says "Use in Well Ventilated Area" and proceed to inhale fumes from the spray. Once your hair has achieved a proper state of green (and/or you begin to feel dizzy) proceed to Step 3.
STEP 3: Act like a Leprechaun. This can include any number of activities including dancing a jig, frolicking in clover or simply approaching anyone who will listen and loudly stating in your best Irish accent "Aye aim a Leprechaun!!!"
*I have no idea why it's called Jaguar Green as I have certainly never seen, heard of, or imagined a neon green Jaguar - either the animal or the car. Feel free to speculate the color naming system on your own.