I replaced the battery, but the screen now only displays 1 color (Red)

"Hello there!

Probably the flex or connector on mo-bo is faulty. As far as I know Apple makes those parts very thin and fragile to avoid messing around by “unqualified personnel” (if only they would be qualified enough and not only using predesigned tools).

Check if the connection is proper for your LCD flex. Clean with IPA, check for bend pins and check if the connector is not bend itself during unplugging. Alternatively check on different screen."