What is a "Dual-Color OW Battle"?
"Dual-Color OW Battle" refers to three other world encounter cards from the Curse of the Dark Pharoah expansion (both the original and revised versions) that are actually two colors - they'll match either color. The "Battle" part, and the reason they're worth keeping statistics on, is that if you're in a certain location and draw the card, you'll have to fight an Ancient One. (If you're somewhere else, you get something much less scary.)

If you don't care about spoilers, here they are: one two three. They're linked from the Curse of the Dark Pharoah section here.