The cost reduction ability reduces only the generic mana in Brine Giant’s cost. The colored mana must still be paid.
24-01-2020
wotc
To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Brine Giant). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
24-01-2020
wotc
Once you announce that you’re casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can’t try to change by how much Brine Giant’s cost is reduced.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.