3rd rank guardian
The lowest rank of guardian angels are typically tasked with taking care of new angels. It is in large part a teaching and mentor job which requires patience and good people skills. These angels have a plain, golden halo.
2nd rank guardian
These guardians are most often stationed outside Heaven where they can monitor mortal worlds, for example if there's a risk of breaches in their dimensional barriers. Sometimes they're tasked with keeping an eye on individual mortals if they have in some way posed themselves as problematic for Heaven. 2nd rank guardians have a glowing, golden halo.
1st rank guardian
The highest ranking guardians practically take care of the same jobs as the 2nd rank, only they are sent to more high risk or problematic situations. Unusual mortals - or angels - are also typically put under their supervision. For instance, this could involve rare cases of "psychic" mortals, who are able to sense and communicate with beings outside the mortal plane.
1st rank guardians have golden, glowing halos with symbols resembling an all-seeing eye. The halo changes the color of their wings to be brown with various markings.