Materials are represented by patterns in vector based rendering techniques. This message will display if a material in the plan has a problem with its pattern.
To reset walls to their default materials using the material painter choose the option Use Default Material at the bottom of the Select Library Object dialog.
There is not a defaults dialog for stairs. Instead, the default materials for stairs are specified in the General Material Defaults dialog.
Reasons why materials would flicker include plan placement, large terrain, materials occupying the same space and video card.
In Chief Architect, images can be added to the Library Browser for use in any plan using the Copy and Paste Special option.
The Material Painter modes help you specify exactly what is painted when you use the Material Painter tool: Component, Object, Room, Floor, Plan.