flashing the background color might be what you need.
Oh yeah, great!
I needed a little bit of KISS-thinking :)
Then you can use a different color when you flash it from "outside".
I don't even need that.
So I could just implement my [control_bang] abstraction like this:
[r $1] | [pd generate_a_pair_of_color_messages] | [s $1-if-receive]
assuming that my [bng]s send directly to the target <name> and receive from <name>-if-receive