Shiffrin clinches 99th World Cup win in Gurgl slalom
US skier Shiffrin said she felt the pressure as she went off last as leader.
"I really was nervous on the top," she told Eurosport at the finish.
"I could hear all the other women going down and their teams cheering and that always means they have really good runs and it was getting really sort of darker," she said.
"I was like: I don't think it's happening today. Push! Push! Push!"
Shiffrin stayed in control as she recorded the fifth-fastest second run in 49.14 seconds for a total time of 1min 40.22sec.
That was enough to finish 0.55sec ahead of Colturi, who represents Albania, and 0.57 ahead of Camille Rast of Switzerland.
"It feels really satisfying to have a really great run down that slope," said Shiffrin. "What a wonderful day."
Both the 18-year-old Colturi and the 25-year-old Rast recorded their first World Cup podium finishes.
Swiss veteran Wendy Holdener, who was second after the first run, slipped to fourth.
At 29, Shiffrin, has 13 more World Cup wins than the most successful man, Ingmar Stenmark, and 17 more than the second woman, compatriot Lindsey Vonn.
Her 62nd slalom victory on Saturday sets her up to complete her century across all World Cup disciplines next week in the slalom and giant slalom in her homeland in Killington, Vermont.
"I guess there's pressure around it," said Shiffrin. "I'll try to ignore it. If it happens it's wonderful. If it doesn't it's nothing to cry about in the wider scheme of things.
"I'm hoping to have a really good performance before my home crowd."