Red can only be used for replacement tags where the original number isn't known, or where the animal isn't on its holding of birth. This is why red tags can't be used for management purposes, or as match-up tags.
Match-up tags can be any colour (except yellow, red or black. (black is reserved for animals that have an electronic rumen bolus)), and the number on them must match the number on the official yellow (EID) tag in the other ear.
However, in Scotland I think that the official defra tags don't have to be yellow.
This is what applies to sheep and goats, anyway. Cattle is similar in so far as the official defra tags are yellow.
This is why I'm guessing farmed deer have yellow tags, but I might be wrong.