The blue color indicates there are exposed back faces. In the case of the wall, it’s because you’re missing the near face of the 3D shape. If you zoom in, you can see that.
There are other places where the faces are reversed. You should not be seeing blue faces in the model on 3D shapes. And painting the back faces with colors and materials isn’t an adequate solution for reversed faces.
I see. thanks! so how did you revert to the non-colored view to see that?
I’ve always wondered why there were two different shades that sometimes didn’t make sense to me. I colored /textured things just to send a screenshot to my client. I’ve never used either of those tools before.
I set the face style to Monochrome (View>Face Style). this turns off materials without deleting them. I would suggest you generally work in the Monochrome face style as a habit and avoid adding materials until the model is at least nearly completed.
went back to look at the drawing. I think I deleted that face on purpose right before taking the screenshot because the back of the shelves and the blackboards have no depth, so the color/texture gets all mixed up with the wall color. inept shortcuts taken in haste.