Catalina Usme scored on a 30th-minute penalty kick after Shim Seo-yeon was whistled for a handball, deflating the Korean side.
"The penalty was a little bit harsh because the ball was really well-struck, at speed, at distance, I don't know where Seo-yeon would have had to put our arms," said Bell, on the shot that glanced off the defender's arm.
"I just wondered why that was not reviewed. I've watched all the matches so far in this tournament, you've had five minutes with review at times (when) nobody knows what's going on. This time it's a clear penalty. So, then you go down (a goal), and then it's very difficult against a very strong Colombia team."
Linda Caicedo, an 18-year-old striker for Real Madrid, added the second goal for Colombia, who dominated for most of the afternoon at Sydney Football Stadium, easily slicing through Korea's defence.
Bell said his team made it too easy for Caicedo.
"Obviously Linda is a really big talent but we just gave her too much space. I used to play fullback, so I would just slide tackle," said the British coach, who was hired after Korea lost all three matches at the 2019 World Cup.
"We had the options to do that also before the second goal. You cannot just shadow a player. We need to be stronger in the one-v-one situation. Not going to say you can stop them every time. But definitely we could have just slide tackled. No foul, win the ball, ball was free. Job done. You're a defender, defend."
The 61-year-old Bell played two seasons for Leicester City. His coaching resume includes numerous pro clubs in Germany and England.
American-born Casey Phair became the youngest player ever to take the pitch in a World Cup game when Bell sent the 16-year-old on in the 78th minute. Bell said she deserved a chance to play, and her presence on the pitch was a message.
"She's trained really, really well, as good as anybody else," he said.
"And it's also a signal. That's the future. Casey is the future. We need strong fast players with the physicality. I think that's a lesson for us all to learn. In Korea we need more intensity ... we're looking for those type of players. I think Casey is one of them, that give us that energy and that power."
South Korea next face Morocco in Group H on Sunday.