Yeah that's correct: every action card shows exactly what happens when you roll a certain number of symbols. Not much ambiguity, really.
The blue dice are the basic pool that you roll.
If your stance is on the Reckless or Reserved (?) dice, then you add one or more Red or Green dice to your pool (respectively).
The Red and Green dice are the same as the blue (hammers, eagles and blanks), except they also seem to have some extra symbols, like Hourglasses on the Green dice (which represent extra time taken in performing the action) and skulls on the Red Reckless dice (which represent misfortune or bad stuff because of acting without proper planning).
White dice I suppose are added for every Fortune Point added from the Party sheet. THe players might decide that the Elf REALLY needs to make this one glorious shot, so they decide to spend a few Fortune Points to give him a handful of extra white dice.
The black dice are the opposite of Fortune: perhaps the wizard rolled badly on Tzeentch's Curse and has to add a handfull of Black misfortune dice to all his actions for the rest of the day. Or something.
Or maybe the black dice are added due to unfavorable conditions, like criticals, harsh terrain or weather?
What were the yellow dice for again?