You’re seeing shading, which helps you make out a 3D object on a 2D screen. You can adjust the darkness/brightness of the light and shaded areas in the Shadows palette. You can also turn shadows on/off and toggle if the sun is used for shading. Play with those to see if you can get it more to your liking. But please remember, shadows are a part of life and this type of thing happens in real life to a certain extent, naturally.
The other very real possibility is that you are not coloring the walls properly, and we might actually be seeing two different materials. It’s hard to guarantee what’s going on without actually opening the model itself.