The only rule I think I ever internalized after six years in the SCA--and designing heraldic devices is one of Tristan's specialties, so I heard a lot about it--was colors can't touch colors and metals can't touch metals. I think. So the [medieval] herald would have a problem with red on black since they're both colors. I thought the [medieval] herald would have a problem with Finrod's crest since yellow touches green, but I think the yellow is actually supposed to be gold--metal!--which makes it okay. However, symbolically, I think red and black are a potent combination; I could go on at length about what a choice of black over white, silver, or gold might mean for the House of Feanor at this point, but I'll spare you. ;)

GRRM's discursions on heraldry are always a signal to me to skim! :D It's weird 'cause I liked heraldry as a kid ... the SCA has soured me, I guess, on this as with many things.