Glamorgan started the day on 228-4 and milked 11 overs of spin before the new ball, Ingram and Kellaway moving from the 50s to the 70s.
There was no change in Gloucestershire’s fortunes with the harder ball as the pair kept the scoreboard ticking over without a chance.
Kellaway, 21, showed mature judgement as he reached the second hundred of the season and his fledgling career off 180 balls, while the ever-serene Ingram took 176 balls to notch up his 16th ton in 59 first-class matches wearing the daffodil badge.
After a superb first session for the batters producing 137 runs, Gloucestershire slammed the brakes on after lunch with Ajeet Singh Dale and Zaman Akhter alternating overs of mainly short-pitched bowling from the River Taff end, while Van Buuren plugged away at the Cathedral Road end.
Ingram was first to crack as he upper-cut Akhter to third man, while Singh Dale induced Kellaway to pull to a leaping Miles Hammond, and the visitors had a second bowling point while Glamorgan stalled well short of a fourth batting point that had looked likely.
Cautious batting from Cooke and Timm van der Gugten (33), together with some negative bowling, saw the tempo drop in the afternoon though they put together a stand of 82.
Cooke eventually got going before Van Buuren removed him and Andy Gorvin (22) to earn reward for his perseverance.
Gloucestershire openers Cameron Bancroft and Joe Phillips could both have been run out in the first two overs as both were guilty of poor calls.
But the pair settled down and blocked out the closing stages to give their side a decent chance of a draw.
Glamorgan’s Ben Kellaway told BBC Sport Wales:
“To get a first hundred at home is always nice and then to back it up and get us to a nice position in the game is a really good feeling.
“It was a bit of a tricky position to come into (121-4) but having Col at the other end helped me, we spoke about managing the first few overs of your innings and getting used to that pitch, then once you got through that, you could open up spots to scores runs.
“I first came across Colin when he worked with me in the academy around five years ago, and for him to be by my side during my development has been really crucial.
“They came with a plan after lunch and executed it well, with the Kookaburra ball you have to follow a plan for a long time so we’ll try to follow a similar suit, hopefully spin comes into it more.”
Gloucestershire’s Graeme van Buuren told BBC Bristol Sport:
“That was a long day on one of the hottest days of the year so far, but our ergies didn’t drop. We came up with some plans, bowling around the wicket, over the wicket, bumpers, and we stuck to it well while they batted well.
“It was an awesome partnership, the old goat (Ingram) still keeps on producing, and Kellaway is a really good young prospect.
“But we were really proactive and kept on coming, hats off to the two fast bowlers (Singh Dale and Akhter).
“The task is pretty simple for us, we have to show some application with the bat.”
