I'm a bit worried that some part icons will cover up the edges or differences in their backgrounds... particularly for icons like the current Electromagnetic Damper, which is a big square with stuff over top. The armour plating or fuel parts might also be problematic, depending on the sizes of things.
That's the reason some icons will need some minor shrinking. It's already done with the armor plating(in the mockup), but still needs to be done for the damper. The tanks should fit in fine(into the internal ones).
To differentiate between external or internal parts, we have two options , that I see - do it by shape, or by color. I'd prefer doing it by shape for a couple of reasons, that all have come up in previous UI discussions and I don't need to repeat here.
Doing it the way shown in the mockup, the player has to look out for the corners - are there any or are they cut off.
Is there a problem with the spacing between icons in the parts list now?
Well, there is none. It works all right with just the current icons, because they are of different shapes and usually don't extend to the borders. But when you put the gray icons behind, all icons will touch, which looks a bit weird.