Tatis, whose team was victimized by a grand slam from Matt Franco that propelled Atlanta to victory Monday, took starter Damian Moss over the right field wall with two outs in the sixth to give the Expos a 5-2 lead.
"That was another big hit, we're starting to get them from him and others," said Expos Manager Frank Robinson. "That's what we've been missing in all these games we've lost."
Colon (1-0), acquired from Cleveland in a six-player trade Thursday, gave the Expos everything they expected when they traded for the 10-game winner. The righthander went seven innings, allowing two runs -- one earned -- and five hits with a walk and six strikeouts.
"Colon did a fabulous job," said Robinson. "Like a true pro, he goes after the hitter. In the seventh, he reared back and got something extra. He threw one 98, 99 (miles-per-hour)? I didn't see that one."
Colon's worst inning was the first.
After a leadoff triple by Rafael Furcal, Julio Franco lined a playable drive that rightfielder Vladimir Guerrero shied away from after possibly losing the ball in the lights. Franco went all the way to third and scored on a grounder that shortstop Orlando Cabrera fired over the first baseman Andres Galarraga's head for an error.
But Colon, and those behind him, settled down thereafter. After the second, he did not allow a baserunner to third. He completed seven innings for the ninth time in his last 12 starts, throwing 77 of 108 pitches for strikes.
"In the first inning, I felt very nervous, even warming up in the bullpen," Colon said through an interpreter. "After the first inning, everything just went away, it just felt great."
Moss (4-3) cruised until giving up a leadoff homer to Michael Barrett in the sixth. Things deteriorated further for the lefthander as Jose Macias and Vladimir Guerrero singled and Wil Cordero walked before Tatis hit his sixth career grand slam.
Atlanta's second loss in the last 13 games ended its five-game winning streak.
The Expos ended a three-game skid and pulled within 8 1/2 games of first-place Atlanta in the National League East Division.