There are a few different gums that I use for my dark colors. Mainly grape bubble tape or by mixing different flavors together. Like strawberry and sour apple gives me a nice rich brown color. Also, in the UK they have cola flavored gum nice and dark. Oh yeah, my personal favorite for chewing is Blackjack and it tastes like black licorice and is dark gray. I am a purist when it comes to my portraits and I only use the 'natural' inherent color of the gum, never any paint or dye – just chewed gum on plywood sealed in epoxy resin.