Okay, I have never made a custom object before, but this is something I need and I'm sure others would appreciate.
I prefer to play The Sims 2 over The Sims 3, but one problem with the garage door in The Sims 2 is TOO BIG, the one in Sims 3 is just right if one could just turn the existing garage door into a 4 tile window - that would allow the flexibility of the garage door in Sims 3 (although animations would be lost for a decorative door).
So how do I clone the garage door, turn it into a 4 tile window (the existing door appears to be 4 tiles wide centered on 5), and have it so the faux garage door takes the recolours of the original door?
You can check out Numenor's Wall Windows . There's a neat part that makes the garage door glass.
As far as cloning it, I usually check the CEP documentation first, to see if there are any special instructions for the object I want to clone. It's also a good idea to scan the forum threads here at MTS to see if anyone has experience with whatever you intend to attempt (in this case- cloning garage doors).
To make a Faux garage door that's really a window, you would need to clone a window, replacing the window mesh with your clone of the garage door mesh, and make whatever personal changes you want to the mesh appearance. Generally considered the standard for learning to make windows is this tutorial by IgnorantBliss. Then you would need to make it a 4- or 5-tile object, using the techniques taught by Echo here and here .
It would be even easier to just make it a purely fake and decorative piece by cloning a statue or picture and using your garage door mesh. This way, you won't have to worry about making the window functions work right.
Finally, to make your new object take the colors of the game object, you would need to follow the technique outlined in JWoods' repository tutorial .
I wish I could have given you a simpler answer, but your proposed project is knocking on the door of being fairly complicated,; thus it will require a little learning to get started. Hope you can get it made the way you want it.