The neon green and maroon stripe make the covers look ugly. Those colors clash and need to be changed to something that will blend better. And ditch the stripe--I don't see popular covers using that.
The first cover needs a star background.
The second cover the background atmosphere matches the ship color so they tend to blend in too much rather than creating interesting contrast. I kind of want there to be lights down there or something to indicate a populated planet rather than random background gasses. It's not bad as it is, but it could use a little more. Or, if that's a nebula, then it could use some back lighting to filter through the less dense gas clumps.
The third cover, the planet is another unlit planet. I like how the darkness contrasts with the ship yet they are both the same color. However, what's with the atmosphere coming off the planet on the left side? Here I'd expect to see star background of space, not blue stuff emanating from the planet. If that's supposed to be a nebula, then make it a different shade that compliments the blue but contrasts enough that we can tell it is a nebula. Also, I'd add city lights to the planet unless it is really supposed to be uninhabited.
There's too much sameness in color with the ships and their backgrounds in covers 2 and 3. This is why they seem boring even though the ships themselves are very nicely detailed. Add a touch of another color and these should perk right up.
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