This is the opposite of a red card.

Green/white maybe.

Fixed, more red now.

How is this more red? Because you added an out? That just makes it a better design, not a Red card. "Creatures can't attack" is not red. It's the antithesis of red. Red gets "Creatures attack each turn if able." Preventing creatures from attacking is squarely in White's color pie.
The +1/+1 counter ability could be either green or white, but that, too, is not red.

Why do you think +1/+1 counters aren't red? I think the idea of building up to a giant vicious battle is very red, even if the can't attack idea is a color pie stretch. Red has almost 30 defenders.

+1/+1 counters can be in all colours. (Rage Forger, Markov Blademaster etc). Putting a +1/+1 counter on everything is... normally white, possibly green, but primarily fiddly. Put charge counters or growth counters on this card instead, and then say "All creatures get +X/+X, where X is the number of charge counters on ~." (Though that won't work after this gets destroyed... hmm... maybe make the "destroy ~" effect also say "put X +1/+1 counters on each creature".)

Preventing things from attacking until a condition is met is not normally red. (Though individual red creatures can certainly not bother attacking until a condition is met: Slumbering Dragon, Goblin Mutant etc.)

I think this would be better as a white card though. White is very much a colour of armies and battles, war and warcraft, and "building up to a final apocalyptic conflict" is probably slightly more of a white concept than a red one. Plus mechanically you don't have to stretch to justify preventing combat in white.