Step 1: What You Need
Glassware (mug, pint glass, etc. Appropriate for style of beer used)
Pale Beer (Something clear and lightly colored, no reds , hazy, dark, etc.) we used PBR, it was cheap and available.
Green Food Coloring
Step 2: Coloring Your Beer
To ensure your beers are evenly colored dye all of it at the same time. I poured about 2 drops of McCormick Green Food Color directly into a 24oz (.75L) can of Pabst Blue Ribbon (about $1.50 a can), and the color was just about perfect.
Optionally, you can use a pitcher to mix more beer with more precise control of the color, this works great if using homebrew from a corny keg or beer from a growler.