Rangers have secured all three points at Ibrox, making it nine out of nine.
It was a tough afternoon for St Johnstone who saw four goals in the back of their net.
The visitors did well to keep the game goalless in the first half an hour but it was Malik Tillman who broke the deadlock with a great header - which seems to becoming quite the habit of his.
With just one goal between the sides at the break, Callum Davidson's side would have been hopeful they still had a grip on the game. And that remained the case for 17 minutes of the second half - despite a lot of pressure from the home side.
Colak was the man who stood up the double the hosts' lead in the 62nd minute, though, with super subs Scott Lawrence and Tom Lawrence also getting their names on the scoresheet in the 80th and 83rd minute respectively.
Morelos made an appearance from the bench as he continues his journey to full fitness, with Gio van Bronckhorst making all five changes to give some of his key men a little break ahead of Tuesday's Champions League play off clash.
There were few goal scoring opportunities for the visitors throughout, with the only clear up chance coming early in the first half. It was simply a long afternoon for the Saints who return to Perth empty handed.
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Key Events
Obstruction?
St Johnstone are travelling forward but their advance is stopped as the battle rattles off the referee.
MOTM
John Lundstram has been awarded the man of the match award.
He is no longer on the field but his earlier efforts mean he recieves recognition from the match sponsors.
Rangers rampant
It's been ruthless stuff from Rangers today - they've barely been troubled despite changes to the starting line-up and a St Johnstone side set up to camp on the edge of their own box.
Colak has again shown his worth with a third in three matches but midfielders Jack and Lundstram were the catalyst. They were key in keeping the tempo high and denying Saints the occasional breather.
Tillman looked sharp again and Matondo also proved influential withn some excellent work for the second and third goals.
Both Rangers and Celtic have raised the floor of their squads this summer and on days like this, with five subs still permitted, you can see it paying off.
Defended well
The ball is dealt with well by Hallberg who plays it far away from the danger zone.
Last minute free kick
A challenge from Carey gives Rangers a free kick outside the corner of the box.
Could be dangerous.
Get it out
McGowan launches it long in a bid to get his side out of trouble for a moment or two.
But the ball is collected by the Rangers back line who are simply keeping possession.
Counting down the minutes
St Johnstone are likely looking for the final whistle to be blown now.
A couple minutes of regular time to see out.
A duo
Lawrence and Arfield have even more positive exchanges towards the St Johnstone box.
It seems to be a positive partnership from the Rangers pair.
Considine does well
Morelos is looking to get his name on the scoresheet, too.
But Andy Considine does well to close him down.
An Arfield impact
Arfield has made an impact after coming on.
He has a goal and an assistant since making his way onto the park.
Some going.
Lawrence goal
Tavernier does well to keep possession in the box.
He then finds Arfield who plays in Tom Lawrence before a terrific finish.
Some great work up play before the sub makes it four.
Arfield makes it three
Jack released Matondo with a great pass.
Matondo them finds Scott Arfield who gets the third goal of the afternoon.
St Johnstone chance
An Andy Considine header from a Graham Carey cofner gives St Johnstone a chance but it is narrowly off target.
Rangers also make two
Leon King and Glen Kamara are on to replace Ben Davies and John Lundstram.
Double sub for St Johnstone
Michael O'Halloran and Ali Crawford replace Jamie Murphy and Drey Wright.
Jack has a go, again
The ball falls to Jack following the corner who has a strike from outside the box.
But nothing too dangerous for St Johnstone.
Morelos the target
Matondo clips it in and Alfredo Morelos is his target in the box.
But some more good defending my Alex Mitchell who heads it out for a corner.
Good block
An excellent block from Alex Mitchell denies Rangers a shot on goal after Arfield tries to find Morelos.
Keen for a third
Tavernier does well and chips to ball into Morelos in the box.
But the striker doesn't test the keeper and the ball is out.
Lawrence wants to make an impact
After good link up play the ball is played for Lawrence to chase.
But he can't quite get there.
The crucial second
A pivotal moment as Rangers make their dominance count with a goal that sums up their afternoon.
They've been relentless in their approach to breaking down St Johnstone's defensive ranks and it's persistence that pays off in the end.
Tillman's through ball is just underhit but Matondo refuses to give it up and he gets a toe on the ball to take it away from Wright and into the path of Colak, who instantly spins and drills into the bottom corner.
The Ibrox faithful can breathe a bit easier, although that remains a challenge in the stifling Glasgow heat (not something you get to write very often).
Second half refreshments
Half way through the second 45 the players take a quick water break.
A trio of Rangers changes
Morelos, Arfield and Lawrence make their way onto the field.
Tillman, Wright and Colak depart.
More attacking
St Johnstone are pushing more of their men forward in a bid to be more promising in the final 25 minutes.
More of the ball?
Carey and May link up well down the right hand side as St Johnstone try to keep possession.
St Johnstone need something
Carey stands up Barisic but not for too long.
St Johnstone will need to dig deep.