Recoloring existing improvements may suit the needs as you say, but for a full set of improvements for a new faction it quickly becomes limited.
Are we condemned to always use the same graphics for improvements?
It would be sad, look at the thousands of models of ships available in the workshop, and if there could also be models of improvements available in the workshop?
I don't know how the improvements .png files are created by Stardock. If they are created with a 3D program or if they use a 2D program.
From what I see by observing the graphics used, they must be created in 3D. It’s faster to do it that way and it gives more possibilities.
The 3D object must be positioned at a predefined angle so that all of the improvements have a visual match.
The program needs three .png files, one for the improvement on the planet with a shadow, a smaller without shadow for the construction list and a third of a solid color without shadow when the improvement is in phase of construction.
All this can be done automatically by the program when the user saves his creations.