I have enough money to buy chain mail. The card shows one grey dice. Does that mean it adds 1 grey dice to my existing defense dice?

Yes

Yes.

Pg. 11, "Shop Item Card Anatomy", #4:

Dice Icons: This area lists the dice associated with the item. If the item is a weapon, these dice are used when attacking with the weapon. If the item is Armor , they are added to the player’s defense pool.

It came up as an early question in one of my first games too, and it's easy to accidentally skim over the boxes in the rules, due to "banner blindness" and thinking it's mostly clarifying examples, but it's got some important rules in there.

Also, armor not adding their dice but merely replacing them would be absurd, so you probably reasoned your way to the answer anyway, but it's understandable that you want to make sure. :3

Thanks! I'm using the road to legend app and it doesn't explain a lot of this stuff.

yep. Add another Grey die to your defense pool. Then watch helplessly as the die always land on blanks.

Hey now! Those grey dice don't always land on blanks. They always roll lots of shields during attribute tests! Oh wait..

My defence dice sometimes roll really, really high... usually when the OL rolls an X on his blue die.

Thanks! I'm using the road to legend app and it doesn't explain a lot of this stuff.

Yeah, Road to Legend assumes you are already familiar with the base rules. Don't sweat it, though. I don't think anyone gets it 100% right their first play through. Heck, I had nearly finished my second campaign before I saw the rule that anyone can spend surges to recover fatigue on any attack.