Inside faces are red

For one of my boxes, the entire underside is red.
I made a similar box with only two holes on top and there is no redness.
What could this mean and will it cause problems if I try to, say, export to DXF?

EDIT: I see this is just a colour scheme. Does anyone know how to change the colour scheme to the default (white outside, gray inside)?

Looks like you’ve got a style selected that uses red as the back face color and your model is either missing a face or at least there’s no thickness to your object.

Pick a different style. It won’t fix the exposed back faces but they will have a different color. Or edit the style and change it.