LOTRO has a color description for each level group:

Grey - Enemy is 9 or more levels below you (you get no XP for the killing)
Green - Enemy is 6-8 levels below you
Light Blue - Enemy is 3-5 levels below you
Blue - Enemy is 1-2 levels below you
White - Enemy is the same level as you
Yellow - Enemy is 1-2 levels above you
Orange - Enemy is 3-4 levels above you
Red - Enemy is 5-7 levels above you
Purple - Enemy is 8 or more levels above you

Ranks

Swarm (Green): some nature mobs, bats, roots.
Normal (Blue): wolves, humanoids, these is the common group.
Signature: the one of a kind… harder to kill but still soloable.
Elite (Eye of Sauron): hard to kill.Require special skills combinations or small fellowship.
Elite Master (Eye of Sauron + 4 Spikes): hard to kill. Requires Fellowship.
Nemesis (Elite + 6 Spikes): harder to kill.
Arch-Nemesis (Elite + Flaming Eye): Nazgul and other creatures.