Material changes color

[micheus]

The problem isn't in the floor. It's in the wall.
You didn't post any wires of the scene, but it seems you are using a low amount of polygons. It's a common behaviour for OpenGL viewing. It's probably to the way the normals are calculated. I'm going to quote a explanation I found (better than I could give you ):

This seems to be the case to me on the screenshot, the shading goes weird because the normals are smoothing faces which are pretty much at straight angle to one another, and consequently the renderer is trying to interpolate between 'in light' and 'in shadow' along what's really a single, flat polygon. (ref. link)

Check this image:

As more vertices your object have better will be the shading.
In the first pair (top/down) the wall is connect to the floor since I extruded the tiles; In the second pair the wall was just separated from floor; in the third some clean up was made in the wall bottom.
In the last pair we can see a better result by using hard edges.