Lars Eller and T.J. Oshie scored for Washington (3-2-2). Ilya Samsonov made 19 saves in relief of Braden Holtby, who was pulled in the first period after allowing three goals on Colorado's first three shots.
"We had a discussion about it between [the first and second] periods and we responded accordingly," Capitals coach Todd Reirden said. "Battled till the end there and have a call not go our way, but you're not setting yourself up for success with starting that way."
Johnson made it 1-0 at 3:42 with a shot from the left point.
Colorado went ahead 2-0 when Zadorov reached back for a pass from Rantanen on the rush, turned, and scored from the slot at 6:00.
Kadri scored with a shot from the left face-off circle at 7:54 to give the Avalanche a 3-0 lead and send Holtby to the bench.
"You've just got to find a way to help the team win," Holtby said. "Tonight, I didn't do that, and it's on me to make sure I improve. It's frustrating and I've just got to get back to work and work harder and find a way back to that mindset that success comes from."
Rantanen knocked in a loose puck from the right circle on the power play to make it 4-0 at 9:25.
"That was our best period of the year," Bednar said. "We found our offensive game, we found our puck patience, we were snapping the puck, executing, and we matched that with real good defending details as well."
The Capitals made it 4-1 at 6:56 of the second period when Ovechkin deflected a clearing attempt to Eller, who was alone in front of Grubauer.
"We tried to bounce back, and I think we did a pretty good job," Ovechkin said.
Oshie made it 4-2 when he deked around Grubauer and slid a backhand shot in at 15:28 for his fourth goal of the season.
Samsonov turned over the puck behind his net, and Tyson Jost fed Nieto for a Colorado goal that made it 5-2 at 4:24 of the third period.
"I think in the third we started to play a little bit more simple, chip it out and make some easy passes instead of forcing it, and then away we go, 5-0 that's a pretty good start," former Capitals forward Andre Burakovsky said.