|static NamedStyle *||default_style = &::style|
You can control the color of the circle with color() and the color of the dot with textcolor(). You can make it draw different colors when turned on by setting selection_color() and selection_textcolor() on the widget (these are ignored if set in an inherited style()).
If you want you can make any other button act like a RadioButton by doing type(Button::RADIO) to it. Be sure to lay out and decorate your interface so it is clear to the user that they are radio buttons.