Forced assimilation could be a major cause of internal conflict
The correct answer is - temperature.
We see the stars on the night sky in multiple colors, and the variation in the colors of the stars comes from the differences in temperature. The cooler a star is, the more reddish it appears, the hotter the star is the more blueish or whitish they appear. From cool to hot, the stars can appear in the variations of red, orange, yellow, green, blue/white colors, and it is an easy indicator of the temperature of them.
