[-----●------] 70% (checkbox: [✓] Show fill)
Newer versions of SketchUp include a "Section Fill" feature that covers the hollow parts of a cut. To adjust visibility : Click the Display Section Fills icon on the Section toolbar. To adjust color/opacity Modeling Settings (the last icon, a small cube). Section Fill Color sketchup section plane transparency
You have a material applied to the section plane itself (accidentally painted with the Paint Bucket). Fix: Select the section plane, open the Material panel, and click the "Default" material. This resets the plane to use the Style settings. open the Material panel
: Set your Tags (Layers) to a color with low opacity. sketchup section plane transparency