[Not-a-bug] What is the configuration setting for terminal bell color?

[Kabouik]

I am sorry, this is not a bug but I see no Github template for simple questions. I am very pleased with SwayFX which I only recently discovered. I use a terminal email client and noticed that SwayFX applies a tint to a terminal window when the terminal bell was triggered (i.e., when I received an email in that client), see the left window here:

However, I can't find in the documentation what is the setting to change that colour. It should not be in my terminal configuration since this effect didn't exist with Sway; unless it was there all along but was not compatible with Sway?