System color (Stellaris)

The question torments me for a long time. Background - fact:
1. Unknown system - no name
2. Known, but not studied - Name is gray
3. Known and studied - Name is white.
4. If there is an outpost - Name in a frame.

Question! Why does the frame change color? If my planet is there, the frame is green.
If the outpost is Red, Gray, White, Green. And it is constantly changing.