configure

Tcl/Tk

#!/usr/local/bin/wish4.2

button .b -text "Hello World!" -command {.b configure -activebackground blue}
pack .b

ruby/Tk

#!/usr/local/bin/ruby

require "tk"

$button = TkButton.new {
  text 'Hello World!'
  command proc{$button.configure('activebackground', 'blue')}
  pack
}

Tk.mainloop