How to config ColorButton in Poly/ML with Giraffe

Question

Now I have created a ColorButton, when clicked a color picker pops up and the user can pick a color.

val color_button = Gtk.ColorButton.new()

At that time, I wanted to directly set the background color of the button to the color I specified (such as red). Then, when the button is clicked, it will directly output the value of the background color set by the button, instead of popping up a color picker.

How should I write the code? According to the gtk documentation, the compiler keeps reporting errors.