Both PSTricks and TikZ/PGF support polygon options that
have the effect of making the polygon appear transparent. For
PSTricks, keywords opacity and transpalpha have
both been used, with the correct one depending on version.
TikZ/PGF uses opacity only.
When transparent polygons are in the foreground, objects behind them
(drawn earlier) are visible with color subdued and tinted. The hidden
surface algorithm of sketch works well with such transparent
polygons.
Note that cull=false must be used for rear-facing polygons to be visible when positioned behind other transparent surfaces.