I never saw this explained in the tutorial, and it looks like not a lot of people even know about it, so I thought I'd explain it here.
Yellow Stars/Class G
These are the default star systems you can travel to from the start of the game. Mostly basic resources can be be found on the planets present within them, and a lot of the planets are barren.
Red Stars/Class K
You need at least a warp reactor sigma to travel to these stars. Planets will have slightly rarer resources on them, and more planets will be habited with fauna and flora.
Green Stars/Class E
You need at least a warp reactor tau to have the ability to travel to these systems. They contain even rarer resources and even more planets will be tropical/lush.
Blue Stars/Class B
You need a fully upgraded warp reactor (theta) to travel to these systems. Planets will have a chance of having even the rarest resources on them, and a higher chance of being lush with plentiful plant and animal life.
In my experience, by default the waypoints to the center, Atlas interference, and black holes only traverse the yellow stars, so by following the path you will only be seeing the lower tier systems. I think it's a way to reward players for venturing off the beaten track, but I'm not certain.
Keep in mind, this is only information from my experience. But to me, it seems that the higher tier systems have a higher chance of having more lush planets with rarer resources.