Split layer to edit each object seperately

Hey everyone! I am new to Unity and this is my first post here.

I imported an .obj file into my project. The file is taken from OpenStreetMap using OSM2World program, in order to have a 3D city.

The file consists of several layers (Forest, Road, Building etc).

What I wanna do is split the 'Building' layer into more layers, one for each building. The reason I wanna do it is so that I can edit each splitting (for example, apply different colors/textures to each building).

As you see in the attached picture, when I choose the 'Building' layer, all buildings are selected.

Any ideas? How can I edit each building separately or split the 'Building' layer?

 

If you're just a beginner the best option for you would be to use a 3d modelling program, here's how it can be done in blender: link

If you want to do this in unity, start reading the documentation of Mesh class.
Steps would be something like:
1. Create empty mesh.
2. Select first triangle of the original mesh, copy the triangle and it's vertices to the new mesh.
3. Search for all triangles that share it's vertices, copy them to the new mesh.
4. For each of these triangles do the step 3.
5. Remove found triangles from the original mesh.
6. Go to step 2.
It would be a nice introduction to mesh manipulation for you, but if it's just a one time thing I don't think that it's worth the effort.