AWS Blue/Green Postgres - default_transaction_read_only not applying for Green deployment

On a green instance the value is set to read_only so that you do not make unintended changes to green that would break logical replication to green. You can however change the value in a session variable if you need to make writes to green.

https://jkatz05.com/post/postgres/postgres-read-only/

You prepare a new parameter group and set defalut_transaction_read_only to 0 for that parameter group. You can then apply the parameter group to the Green environment and restart it to maintain the database on the Green side. Alter System is not available on Aurora.