Unlit scene (default lit objects) difference in color depending on angle

Hello. I want to make a quick game and am trying to avoid playing around with lights so I started to make every level without any light sources and use default lit textures on the objects. Now on one important place I see that when one of my walls is a roof (flat 90 degree) the color is maybe 5 times darker than the original texture. I also notice this happens with the default texture on the same objects (if I remove my picture from them).
If I rotate the dark object 90 degree (make it straight wall again) it becomes bright again.
I want them same color all the time without any changes depending on angle. How can I do this?
On the pictures is same object copy pasted, one time one is flat one time they are both straight facing each other.