Haha cute costume idea!

The answer below is good but if you want another alternative, here is something a little different:

Head: Cap (just a square white hat)
http://www.lynchs.com/images/1895.jpg
^ Like in the photo above without the brim.

Face: Green face paint.

Torso: Wear a green long-sleeved tight shirt and on top a green shirt with the logo screenprinted on so that it wraps around you. This is if you're really pressed for time. Or do something like the below:
http://www.killercoke.org/rbccan.jpg
http://www.komar.org/bbq/bbq_grill_recipes/mcdonald-french-fries/coke-can.jpg
http://farm1.static.flickr.com/34/69589022_86b8c08251.jpg?v=0
http://www.thriftyfun.com/images/feedback_image.lasso?id=54785324

Legs: Wear green tights.

Feet: Green fuzzy shoes (that'd be so cute).

thanx, you had the best ideas. :]

NO LONGER ANSWERING QUESTIONS

References/Helpful Sites:

Song Meanings + Lyrics:
http://www.songmeanings.net/

Slang + Etc:
http://www.urbandictionary.com/

Beauty Products Reviews:
http://www.makeupalley.com/

Hair Issues:
http://www.livejournal.com/community/madradhair

Gender:
Location:
Member Since:
Answers:
Last Update:
Visitors:
Favorite Columnists