If I have a Double Colorless Energy attached to Tangrowth, will its "Grind" attack do an extra 20 or 40 damage for that energy card?
It will do an extra 40 damage. Grind looks at the total amount of energy attached, not energy cards.
Source: Call of Legends FAQ (2011-02-10), PUI Rules Team (2011-02-10)

Relevant cards

Note: All applicable cards may not be listed below.
COL1_34