De La Rosa (4-4) held the Rockies to one run on three hits, overcoming four walks while fanning five for the Dodgers, who won for the seventh time in eight home games.
Ethier's blast in the third broke a 1-1 tie and he later added an RBI single as the Dodgers collected 12 hits to take the opener of a three-game series against Colorado.
Aaron Miles drove in a pair of runs with a third-inning double and single in the eighth.
Juan Nicasio (4-3) was roughed up for five runs on six hits for Colorado, which lost for the 11th time in 14 road contests.
The Rockies scored for four runs in the ninth, highlighted by Dexter Fowler's RBI single.