Faces and Surfaces in SketchUp have a Front side and a Back side.
SketchUp’s default face materials are:
Front = White
Back = Bluish Gray
Best practice is to keep all visible faces of a model oriented such that the Front Side (white) faces outward.
Face orientation is important as certain export formats and renderers only support single-sided polygons.
You can check face orientation anytime by viewing the model in Monochrome Face Style.
View > Face Style > Monochrome
Change face orientation by using the Context Menu commands.
Right context click on a face and select … Reverse Faces or Orient Faces.
• Reverse Faces
The Reverse Faces menu item is used to flip the front and back sides of the currently selected face.
• Orient Faces
The Orient Faces menu item is used to automatically orient all the connected faces of your
model to the orientation of the currently selected face. This option could lead to unexpected
results when an edge of a face bounds three or more other faces.
Faces and Surfaces retain their Front / Back property regardless of what other materials you might apply.
And each side of a Face or Surface has its own individual material attribute.
Notice when a Face or Surface is selected the Entity Info Box displays two material swatches.
The left swatch is the Front side material; the right swatch is the Back side material.
You can change/edit the front or back face material by clicking on the respective swatch.