This happens when your app has multiple databases:
As part of the provisioning process, a DATABASE_URL config var is added to your app’s configuration. DATABASE_URL contains the URL your app uses to access the database. If your app already has a Heroku Postgres database and you’ve provisioned another one, this config var’s name instead has the format HEROKU_POSTGRESQL_<COLOR>_URL (for example, HEROKU_POSTGRESQL_YELLOW_URL
You can promote your new database to make it the primary database:
pg:promote updates the value of the DATABASE_URL config var with the newly promoted database’s connection string. It also creates an alternate attachment for the old primary database, assigned with a new HEROKU_POSTGRESQL_<color>_URL config var. The promotion process triggers a release and restarts the app.
For example:

heroku pg:promote HEROKU_POSTGRESQL_MAUVE_URL -a your-app

You should continue to use the DATABASE_URL environment variable to connect to your database.