UO Second Age

I don't have any concrete evidence on this just yet, but I am fairly certain that pets of players who are in an O/C guild &/or a guild with wars, should be orange (freely attack-able) by enemy guild members.

This was a pretty effective mechanic against the players running horses --> dismount --> all follow me --> remount. The 1-2 second window of dismount was enough for a quick player to engage/poison an opponents horse, and quickly taking the upper hand in the fight.

If anyone can dig up some info on this before I am able to, please post.

Only if the pet was issued a kill/guard command should it turn the color of the warring faction iirc.

Just check for pictures of dead warring guildmates for proof. The horse will be blue because nobody is telling their horse to "guard" or "kill".

Matron is correct.

Matron is definitely correct.

The game mechanics behind this is actually described in the Reputation System FAQ for flagging in this manner.