The dark blue ring may be indicating the system has un-surveyed habitable worlds. That would be my best guess. I'm afraid that I don't have that option turned on and I don't have any race I control that has that to check. Hopefully someone can give you a definitive answer...Starslayer?
The white ring indicates a DSB-Xr array is present in the system (just to remove that question). I thought somewhere there was a colour code explaination but I could not find it looking in the program just now.
Light Blue -- Uncolonized benign or hostile world (typically superterrestrial)
Light Green -- Sensor buoy network
Red -- Grav survey not completed
Pink -- Unexplored known WPs (which is also what pink dots mean, oddly enough)
Yellow -- Suspected or known but unlocated closed WP
Dark Blue -- ?
The first four I think I got from Steve back when, and the info on yellow rings from you.