Obi says that is the first treble he has had since he was about 16, and it capped an excellent team performance, with Marc Williams again outstanding in midfield.
Williams set up the first goal after only seven minutes, tricking his way past two defenders to get inside the penalty area and from his pull back Aaron Rey’s deflected shot was turned into the roof of the net from close range by Anoruo.
Gainsborough missed a good chance to equalise and Josh Ollerenshaw turned a fierce shot away for a corner, before the Bay made it 2-0 on 41 minutes.
Trinity keeper Mike Emery came out of his area to try and clear the ball, but lost out to Tom Buckley, who crossed for Obi to score into an unguarded net.
Colwyn Bay then dominated the opening 20 minutes of the second half.
Buckley, who also had an excellent game, was twice denied by good saves from Emery before Marc Williams made it 3-0 with a header from a Buckley corner on 57 minutes.
Five minutes later Anoruo got his hat-trick goal with a cracking shot on the run from 25 yards.
Gainsborough got a late consolation goal from a corner.
Colwyn Bay move into the top eight with this win, three points behind fifth-placed Guiseley who they visit next Tuesday.
Delighted manager Frank Sinclair said: “It was important after the 5-0 defeat in our last home game that we put in a good performance and got a good result and so I was really pleased.
“A big part of this season is to start to mirror how we do away, where we seem to be decent and pick up good points, when we play at home.
“The big question for the players today was could they repeat the performance last Saturday (at Leamington) and they done that to be fair.
“We are getting better. The understanding in the team is getting better, the performances are getting better and we are playing some good football.
“I thought once the game had settled down today we played some super football and I was chuffed for Obi because he has been fantastic since he came into the club and gives us something we never had last year with his pace and goalscoring. He’ll be a threat to anybody.”
Colwyn Bay: Ollerenshaw, Olapade, Moss, Anjos, Bembo-Leta, Noone (Haywood 74mins), Buckley, Anoruo, Rey (Hopley 67mins), Williams (Hodgson 82mins), Royle. Subs not used: Sharpe, Saiel.