IIRC, it is a feature that requires to you scour the Dev Guide in 2 places and be aware of subtle differences.

The Dev Guide mentions both bgcolor Property and .setBgColor() Method.

In the "Applicable for" section of the bgcolor Property, it states that the Property is only applicable for a Panel.

In the "Additional Info" section of the setBgColor() Method documentation, under "Additional Info", it states:

Additional info
Contrary to the bgcolor property, which relates to the current background color of a widget, this
method allows setting the colors to which the background color will be set after a state transition
(pressing or releasing a button).

I think I may have logged this with the team in the past as a bug (with regard to the field/property 'bgcolor' not working for a Button as expected but do not recall the response. Maybe the response was the modified section in the documentation???

For future reference on reporting bugs to the team:

The simplest way to report the issue is to create a small, simple XCF that exhibits the problem (you can do this with a home page button) and then document the problem and attach zipped XCF in an email to the ProntoTeam.

You can find that email addy by looking up the user account ProntoTeam on this site.