Light blue dots? dark blue dots?

ok this is a compleately stupid and unimportant question but its been driving me bonkers scince i started this game. when i am in towns or outposts some people have dark blue dots and some people have light blue donts on the compass map. is this just a random thing or does it actually mean anything? i've noticed no similarities such as class, guild, armor, level or anything else i can see with out actually playing the charecter. what do they different colors mean?

dark blue - in a group
light blue - alone

people with numbers over their heads are party leaders. these are the people you want to pester to let you in if youre trying to find a group for a mission.

took me a while to figure this out myself...