Cummings scored after an assist by Pablo Mastroeni in the 11th minute and then scored unassisted just before the break. Colin Clark wrapped up the scoring in the 49th minute, with assists from Cummings and Jacob Peterson.
Colorado (5-2-5) ran its unbeaten streak to 10 matches.
D.C. United (5-3-8) has no matches until a July 4 engagement at Columbus.
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Boston schools pull out free condoms over wrapping complaints