Combo is the number of notes in row that were hit by the player with marvelous, perfect or great judgement. For this reason, every judgement below these (good, boo, miss) is called a combo break. Combo does not change the WIFE score in any way.
When the player plays through 1/3 of a file without losing combo, its color is changed to green. If he also didn't get any greats, it changes to yellow. Moreover, if the player got only marvelous, the color is changed to blue.