Chad Bettis (11-7) allowed two runs, both on solo homers, and five hits with six strikeouts over seven innings in his longest start since April 10.
Giolito (0-1) allowed four runs and six hits in five innings for the NL East leaders. Washington has dropped six of eight.
Arenado now has 31 RBI in August.
The Nationals officially recalled Giolito from Triple-A Syracuse before the start of Sunday's game and sent infielder Wilmer Difo to Double-A Harrisburg. Washington's top prospect allowed three of the first four batters to reach as Arenado's RBI single put Colorado up 1-0.
Turner immediately tied it in the bottom of the first with a leadoff homer.
In third, Arenado's liner cleared the fence in left for a two-run blast and Dahl followed with a drive for a 4-1 lead. All three runs in the third came with two outs.
Arenado's fifth-inning triple was initially scored a three-base error on Harper misjudged a fly ball hit to sunny right field.
Harper went 1 foir 4 a day after he was ejected in the 10th inning of Washington's 9-4 loss for arguing a called third strike.
Washington reliever Koda Glover's wild pitch landed flush on the left shoulder of home plate umpire Mike Muchlinksi in the eighth. As Nationals catcher Ramos simultaneously searched for the ball and checked on the crumpled umpire, Daniel Descalso scored from second base for a 5-2 lead.
Muchlinksi remained in the game.
Colorado won the season series 4-2.
Rockies: RHP Tyler Chatwood (10-8, 3.75), on the disabled list with a mid-back strain since Aug. 16, will have a rehab start Monday for Double-A Hartford at Richmond. Chatwood is expected to go five innings or about 75 pitches.
Nationals: RHP Joe Ross (right shoulder inflammation) allowed one run on three hits in one inning Sunday in his first rehab appearance with Triple-A Syracuse.
Rockies: Colorado returns home to begin a three-game series against the Dodgers. RHP Jon Gray (8-6, 4.61 gets the nod for the Rockies.