What makes Brigitte Ultimate Armor diferent from normal armor?

This differently coloured armor was introduced at the same time as Brigitte, but it is not exclusive to her; it signifies that the armor is temporary and once removed it can't be recovered just by healing.

In addition to Brigitte's Repair Pack and Rally, Torbjorn's Armor Pack was changed to the same colour when Brigitte was released.

Like Lucio's shield, the armor overheal from Brigitte's Repair Pack is temporary and goes away after 5 seconds. When it goes away, it all disappears at once. Other than that, it acts like normal armor, which is discussed in this answer.

The armor from Brigitte's Rally ultimate acts more like the armor her father, Torbjörn, tosses out in that it lasts until destroyed.