Tactics card #3 allows players to pull a die from the core, set it to any standard colour and keep it on the card until usage.
Now strictly spoken when I use this very die it is not directly from the core. Would that mean that in a turn I could use the die on the tactics card AND another one from the core? I guess not os, but just want to confirm.
Where could I find a rule reference for this (or an analog rule/FAQ from Mage Knights)
The official Mage Knight FAQ (http://wizkidsgames.com/wp-content/uploads/mage/MK_FAQ_1.0v2...) answers this with respect to the Mage Knight equivalent Mana Steal tactic. The stolen die is no longer part of the Source, thus does not count as your use of a die from the Source on your turn. The primary purpose of this tactic card is to allow you to use two mana dice on the same turn (one from the Source, and the other from the card). Aside from that, it's only minor benefit would be preventing another player (in a competitive game) from using that die. It would be a very sad tactics card if that was it's only benefit.