There are four different colors for the moons that you will encounter, each one representing the difficulty of that moon. The higher the difficulty, the fewer miners and more enemies there are

The four difficulties are:

Blue: Easy
Green: Moderate
Yellow: Hard
Red: Very Hard

The odds of seeing different moons are based on the game variation you choose:

Variation 1: Blue and Green.

Variation 2: Blue, Green, and Yellow. (Mostly Blue and Green)

Variation 3: Same as above except it's mostly Green and Yellow.

Variation 4: Blue, Green, Yellow, and Red.