"Supporting units give up their normal attack to add their attack dice minus one to the lead unit’s attack. All attack dice are rolled together."

Forests:

"Maximum of two combat dice rolled against a unit in a forest hex (before modifiers)
Unit in a forest hex rolls a maximum of two combat dice (before modifiers)"

First case:  A and B are in a forest and want to make a coordinated attack against C, not in a forest.

1/ Can A roll more than 2 dice thanks to B (B is in forest)?

2/ If yes, can B add more than 2 dice?

3/ If yes to 1/ and no to 2/, does B add 2 for forest minus 1, or : color minus 1, max being 2?

 A and B are red. How many dice are rolled?

A and B are not in a forest and attack C, which is in a forest.

4/ With a combined attack, can A roll more than 2 dice?

5/ If yes, can B add more than 2 dice (target in forest)?

6/ If yes to 4/ and no to 5/, does B add 2 for target being in forest minus 1, or : color minus 1, max being 2?

A and B are red. How many dice are rolled?

Hope this is all clear!

This one should really be more fully explained in the FAQ/Errata document, so I hope you submit it to the rules questions page. 

But here is how we played it:

Basically any unit in a forest or attacking into a forest has only 2 dice, no matter their color, then you subtract dice for assistance in a coordinated attack. 

So it seems confusing and it is somewhat, but I believe that the intention is to pretend like each unit in the coordinated attack is attacking normally individually, while considering terrain (so any unit attacking into or out of a forest would be limited to 2 dice, no matter their rank/color), then subtract 1 die from any unit that is not the lead attacker before rolling all coordinated dice together.

Spot on Ken Toad!  My analysis too.  Just treat the attacks like seperate ones, but resolve as a single roll with additional units adding 1 less dice than they would on their own.  I believe the idea behind the 'coordinated attacks' rule, was to give a bit more chance to 'capturing' commanders and overcoming 'Toughness'.  Normally 2x2 attacks should be more effective than 1x3, but the above situations don't.  Also, seperate attacks could force retreats, leaving the other units nothing to hit.  This gives you the choice to decide which is best.  No one can influence the actual die rolls.  If you are anything like me, I always throw more Blue when I want Green, get retreats at the wrong time and get Valour when it's useless.  I suppose it does all even out at the end.  Cheers!

That was my analysis as well. As for asking FFG directly, I already have quite a few questions awating answers on another game!

I think that the coordinated attack can be considered an attach modifier, so 2 units attacking from forest in a coordinate attack would always roll 3 dices no mater the rank.