I was playing commander on MTGO with General Ferrous Rokiric as my general (fitting, I know). Anyways, I didn't get a Golem trigger when I played Boros Signet.
I expected to because a card with mana symbols in its text box dictate its color. This is related to another curiosity: my opponent had Chitterfang, Squirrel General, and they couldn't target Ferrous with its ability.
I thought because Chitterfang has black mana in its box, it's a black and green spell, and therefore can target Ferrous.
Please help me understand what I'm missing here. Any rules searches I've conducted just talk about Color Identity, and I understand that to a point; but why is Signet not multicolored, and Chitterfang mono-colored?
For purposes of Ferrous, Boros Signet is colorless, notably because it has no colored mana symbols in its mana cost (C.R. 202.2, 105.2c). This is true even though it does have colored mana symbols in its rules text. Thus, Ferrous's triggered ability won't trigger when you cast Boros Signet since it's not multicolored, that is, it doesn't have two or more colors (C.R. 105.2b). A card's color should not be confused with its color identity (compare C.R. 202.2 with C.R. 903.4). See also this thread and this thread.
Also, Ferrous has hexproof from monocolored, so that among other things, it can't be the target of abilities from monocolored sources your opponents control (C.R. 702.11d). In this case, Chatterfang is monocolored (namely, green and no other colors) (C.R. 105.2a), even though a black mana symbol appears in its rules text, so if an opponent controls Chatterfang, its activated ability can't target Ferrous in this scenario.
In general, a card's colors are given by the colors of mana symbols in its mana cost, not also in its abilities (C.R. 202.2). Some cards have a color indicator specifying its colors (e.g., Ancestral Vision) (C.R. 204.2) or a characteristic-defining ability relating to color (e.g., Culling Drone's devoid ability [C.R. 702.114a]) (C.R. 604.3), but neither Boros Signet nor Chatterfang is one of them.
I expected to because a card with mana symbols in its text box dictate its color. This is related to another curiosity: my opponent had Chitterfang, Squirrel General, and they couldn't target Ferrous with its ability.
I thought because Chitterfang has black mana in its box, it's a black and green spell, and therefore can target Ferrous.
Please help me understand what I'm missing here. Any rules searches I've conducted just talk about Color Identity, and I understand that to a point; but why is Signet not multicolored, and Chitterfang mono-colored?
For purposes of Ferrous, Boros Signet is colorless, notably because it has no colored mana symbols in its mana cost (C.R. 202.2, 105.2c). This is true even though it does have colored mana symbols in its rules text. Thus, Ferrous's triggered ability won't trigger when you cast Boros Signet since it's not multicolored, that is, it doesn't have two or more colors (C.R. 105.2b). A card's color should not be confused with its color identity (compare C.R. 202.2 with C.R. 903.4). See also this thread and this thread.
Also, Ferrous has hexproof from monocolored, so that among other things, it can't be the target of abilities from monocolored sources your opponents control (C.R. 702.11d). In this case, Chatterfang is monocolored (namely, green and no other colors) (C.R. 105.2a), even though a black mana symbol appears in its rules text, so if an opponent controls Chatterfang, its activated ability can't target Ferrous in this scenario.
In general, a card's colors are given by the colors of mana symbols in its mana cost, not also in its abilities (C.R. 202.2). Some cards have a color indicator specifying its colors (e.g., Ancestral Vision) (C.R. 204.2) or a characteristic-defining ability relating to color (e.g., Culling Drone's devoid ability [C.R. 702.114a]) (C.R. 604.3), but neither Boros Signet nor Chatterfang is one of them.
EDIT (Jul. 19): Correctness edit.
EDIT (Apr. 15): Correct Chatterfang's name.