SketchUp newbie alert! All due apologies…
My problem: Every time I drag one element with a colored face over another one, the colors glitch out. The example here: I had accidentally deleted the fill in the interior space (showing as grey in first screengrab). Following @Geo’s sage advice, I re-added the fill by redrawing one edge (2nd screengrab; see how the existing color is glitching). You can see what happened when I then filled the grey space with black using the Paint Bucket (3rd grab).
My set-up: SketchUp Pro 14 (trial version); MacBook Pro (2.3 GHz Intel Core i7) running OSX 10.9.5.
Many thanks for reading.