Some specific statuses will be represented by different colors... may be what you are seeing.

Targetted Players may be blue
Aggroed but unclaimed mobs may be the slightly brownish color... its actually sort of leaning towards yellowish/orange I think
Mobs claimed by others will be purple
Mobs claimed by an alliance will be sort of in between the shades of red and purple if I remember right... haven't been in a 16+ dungeon for a couple weeks to verify.
Objects will have a grayish/white ring.