Seattle goalkeeper Stefan Frei made three saves to post his first shutout of the season.
The Sounders improved to 2-0 after defeating expansion FC Cincinnati 4-1 last weekend in the season opener.
Colorado goalie Tim Howard, the 39-year-old former U.S. international standout who has announced this will be his final season, made one save.
The Rapids, who opened with a 3-3 tie at home against Portland in a game that was 18 degrees at kickoff, the coldest in MLS history, struggled to mount much offensively.
The Rapids also played without center back Axel Sjoberg, who was issued a red card in the opener and was suspended for Saturday's match. In addition, fellow defenders Kofi Opare (foot), Kortne Ford (knee) and Tommy Smith (knee) were out.
That hurt the Rapids in the opening minutes, as Leerdam and Ruidiaz each scored for the second consecutive game.
Leerdam opened the scoring in the fifth minute. Defender Brad Smith made a run down the left wing and sent a cross into the middle that deflected off Ruidiaz and a Colorado defender.
A shot by Nicolas Lodeiro was blocked and the ball bounced back to Leerdam, who was positioned at the penalty spot. He put a left-footed shot just inside the right post.
Seattle's second goal was similar to its first.
Victor Rodriguez drove down the left wing and sent a cross that deflected off a defender and to Ruidiaz, who was about 8 yards out in the center of the penalty area. He hammered a left-footed shot over Howard in the center of the net.
The Sounders improved to 18-5-2 all-time against the Rapids, including 11-2-1 at home.