I don't know how I feel about the colours on this one, I think they come across a little tacky. I tried to pick colours close-ish to those of the actual moons (from left to right: Callisto, Europa, Io, Ganymede).
This one looks a bit nicer, I think, applying Jupiter's colour to all of the circles. My head-canon for this one is that it is a later flag, symbolic of the individual cultures of the colonies blending and merging into one.
An all-white variant of the above. I think the Red Spot stands out a little too much on this one.
The above without the Spot. Maybe a diplomatic variant?
All-red. Could be the one flown on warships.
The black background might be a bit harsh; maybe a navy-blue field like on the Australian flag would be better.