Zito (2-0) worked around seven hits and issued only one walk in the 108-pitch performance, striking out four en route to the win.
The left-hander also was potent at the plate, going 2-for-3 with an RBI and two runs scored.
Posey delivered a run-scoring triple in the first inning, opening the scoring, and socked a ground-rule RBI double during the Giants' five-run second.
Andres Torres went 3-for-5 with a two-run single while Marco Scutaro and Brandon Crawford each supplied two hits and an RBI as the Giants banged out 16 hits en route to completing a three-game sweep of the Rockies.
Colorado starter Jeff Francis (1-1) was tagged for seven runs (four earned) on seven hits over just 1 2/3 innings.