i believe it will not be allowed, since things with off color kicker costs or alternate color costs are normally not allowed. The bringers are only allowed in a five color deck. Though as always, edh is a variant casual format, talk to your playgroup.
Cards are only allowed to have the mana symbols of your general's colors. U and B are on the Bringer and not your general's colors so you can't play it.
The General's mana cost defines what color of mana symbols may appear on cards in the deck. If any other symbol appears anywhere on the card, then you can not use it in your EDH deck.
Normally, there wouldn't be a problem with Bringer of the Black Dawn being in my deck, but it has a U and a R in the text box.
Does this still count, even though it is an alternate casting cost and not an actual ability?
To make sure you're clear on this, that is an "actual ability" of the Bringer.
"You may pay WUBRG rather than pay [CARDNAME]'s mana cost" is its text, and its effect is a rules-modifying one, to create an option for alternate payment in the event of casting that card.
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Normally, there wouldn't be a problem with Bringer of the Black Dawn being in my deck, but it has a U and a R in the text box.
Does this still count, even though it is an alternate casting cost and not an actual ability?
To make sure you're clear on this, that is an "actual ability" of the Bringer.
"You may pay WUBRG rather than pay [CARDNAME]'s mana cost" is its text, and its effect is a rules-modifying one, to create an option for alternate payment in the event of casting that card.
Awesome avatar provided by Krashbot @ [Epic Graphics].
Yes, hybrid symbols contain 2 mana symbols in them. If either of them is not in your general's colors you can't use that card.