Easiest is to mix something yellow with something blue. Orange juice and Mountain Dew are good for the yellow, Great Bluedini kool-aid is good for the blue. You'll end up with a radioactive shade of green. If you're using a punch bowl, try this: Get some of those latex gloves, fill them with water or juice tinted with food colouring, freeze. Unmold carefully and float the disembodied hands in the punch bowl. Dry ice and back lighting are good for effect too.
NOTE FROM LINDA GROSS: "If you try the water in the gloves trick, be careful because the fingers are fragile and snap off easily, but that could add to the effect if you drizzle red food coloring on the "wound"!