Other players' names are different colours, yellow/orange for your guildmates, blue for players in your party and so on…
This picture is pretty hectic but you can clearly see at least three different shades of green.
Really bright green, green and a pale green.
What do they mean?
If you right click on the image and Open image in new tab you can see it much clearer.
Best Answer
The very bright green names such as "Illusionary Warden", are player 'pets'. Illusionary Warden is a mesmer's phantasm, while some of the other names may be ranger pets, and some of the names in that color appear to be player names but are actually mesmer clones.
The 'middle' green names are player characters.
And the pale green words such as "Champion of the Gods" or "The Sunbringer" are just the titles that the character whose names they are under have obtained and are displaying.