Red is a slaighter only tag...like the new years lambs
Black is for a Bolus EID
Yellow is for an EID tag.
RED is a replacement tag for any sheep/goat that lost its original tag and their numbers are not known. Nothing to do with slaughter at all.
Yellow is EID (in England) plus another one for the other ear which can be any colour except reserved ones, in Scotland your tags can be any colour except the reserved ones. Mine are all white. (both EID and non-EID)