Gravel - Essentially Useless
As we all know, gravel's only main purpose is creating Reactor Components. Once we have enough reactors to power everything on our ships and stations, gravel essentially becomes a useless resource.
I have three ideas for how gravel could be a bit more useful; one simple, one moderate, and one a bit more elaborate.
This does bring about one downside; as the ice moon Europa does not possess large quantities of stone, constructing bases on Europa would become much more difficult, or would require beginning in space or another world.
If it's decided that Steel Plates shouldn't use gravel, then perhaps other metal resources could, such as Interior Plates, Large Steel Tubes, and/or Metal Grids.
For an elaboration on the color system, the normal color selection screen could be kept, in which using the paint gun on a block would apply the currently-selected color. Changing colors with the bracket keys, such as when placing blocks, could perhaps be kept, as well.
Since concrete and like materials aren't typically welded, building the blocks could require specialized tools for placement and removal. For simplicity, perhaps the drill could be used to remove concrete, though I'm not certain if the game code would allow this.
In the event players wish to use concrete as an alternative ship armor; make concrete blocks roughly 75% the strength of Heavy Armor, but 500% the mass. This should make concrete useful only in surface bases.
If reinforced concrete is to be used, then perhaps a welder would be required to build the framework, then a concrete tool to fill the concrete. As with destroying concrete with a drill, though, I'm unsure if the game code would allow this. (Alternatively, placing down a concrete block could require one steel plate, in which a dedicated tool would be required to add the concrete.)
I'm aware that the Gravel Paths thread exists, but the user who submitted that idea didn't offer any additional details.