Short story where a war is fought between two essentially interchangeable factions. After action members of the opposing sides socialize like typical suburbanites.
"Battlefield", short story by Harlan Ellison; first published as "His First Day at War" in Space Travel, November 1958, available at the Internet Archive. The Black and White armies commute between the peaceful Earth and the battlefields on the Moon:
At day's end, at 1630 hours, the death toll was slightly below average for a weekend. Dead: 5,886. Wounded: 4. Damages: twelve billion dollars, rounded off by the Finance & Reclamation Clerk. The batteries were silent, the crabs back in their depots and pools, the airless dead face of the moon left to the reclamation teams, who worked through the "night," preparing for Monday morning, when the war would resume.
The commuters were racked, and as the Blacks filed into their ships, as the Whites boarded theirs, the humming of great atomic motors rolled through the shining corridors of the commuters. Inside, men read newspapers and clung to the acceleration straps for the ride down.
Down to Earth.
For a quiet evening at home, and a quiet Sunday . . . before the war started again.
At the end one of the dads tucks his child into bed and says something like blues is good reds is bad.
All but Polikushka, the children ran laughing to the dining hall which ran parallel to the tiled front hall of the house. The dark-haired Polikushka clung to her daddy's hand and walked slowly with him. "Daddy, are you goin' to the moon tomorra'?"
"That's right, baby. Why?"
"Cause Stacy Garmonde down the block says her old ma—"
"Father, not old man!" he corrected her.
"—her father's gonna shoot you good tomorra'. He says all Blacks is bad, and he's gonna shoot you dead. Tha's what Stacy says, an' she's a big old stink!"
Donnough stopped walking and kneeled beside the wide, dark eyes. "Honey, you remember one thing, no matter what anybody tells you:
"Blacks are good. Whites are bad. That's the truth, sweetie. And nobody's going to kill daddy, because he's going to rip it up come tomorrow. Now do you believe that?"
She bobbled her head very quickly.
"Blacks is good, an' Whites is big stinks."
He patted her head with affection. "The grammar is lousy, baby, but the sentiment is correct. Now, let's eat."