After I joined together a mirrored joined duplicate into one single mesh, the duplicated mesh becomes purple:
What is going on here? Do I need to be worried about this? When joining the roof with the base of the building, this doesn't happen.
The difference in face colours is an indicator that the direction of those faces are not uniform.
Enter edit mode and look for the side panel submenu called "Mesh Display"
In the normals subheading, select the face icon. Lines (called normals) will start to emit from each face. I've adjusted the
size slider so those lines aren't ridiculously large or small. The thing to note here is that if you don't see the line emitting from some faces, that those faces are flipped.
To make them uniform press cltr+F to get the faces menu and Recalc normals (Or Directly using Ctrl+N)
This makes the mesh faces point in a uniform way, and the colour difference will go away: