Level inside a cube

My level should be inside a cube. Therefore I used 6 individual cubes as the respective cube faces. However, although duplicated to be exactly the same, the ceiling cube is rendered black, when viewed from inside the cube. It has the correct color outside of it. What should I change, so it will have the same color as its adjacent walls? I experimented with light sources, (point and directional) but neither achieves the look I’m going for. I just want them to be visible, not noticably illuminated.

Don’t use static lighting would be one thing.

Another, rather than building it from 6 cubes, make it from one. Turn the collision off and make the material 2 sided. ( You can always put blocking volumes to stop the player moving out of bounds.

In your skylight settings, set lower hemisphere to be a color other than black.

I have no light at all in my scene. No sky light nor any other. I thought this works, since the other walls are all visible from inside. Only the top one is causing the problem.

Doesn’t make sense honestly. I have no idea what your scene setup is like at this point because when I have no lights in my scene, it is pitch black.

Your lighting looks very typical of a skylight with a black lower hemisphere is all I can say. If you truly have no lights in your scene then maybe you should add one or just use unlit materials.