I’ve added support for this, and it will be available in the next release.
Keep your background-colour code in tr_window’s Opening handler. In the new window’s Opening handler, enter:
Super.Opening()
# Set up this window's fields and buttons here.
Super.Opening() runs the parent’s handler on the same window, then execution continues with your field and button setup. You can put the call later if you want the parent’s code to run afterwards, or omit it to replace the parent’s behaviour.
Tracked here: Call parent event handlers with Super.