def _palette_cmd(self, event):
        """Respond to user click on a palette item."""
        label = event.widget
        label.master.focus_set()
        label.master.configure(relief="sunken")
        r, g, b = self.winfo_rgb(label.cget("background"))
        r = round2(r * 255 / 65535)
        g = round2(g * 255 / 65535)
        b = round2(b * 255 / 65535)
        args = (r, g, b)
        if self.alpha_channel:
            a = self.alpha.get()
            args += (a,)
            self.alphabar.set_color(args)
        color = rgb_to_hexa(*args)
        h, s, v = rgb_to_hsv(r, g, b)
        self.red.set(r)
        self.green.set(g)
        self.blue.set(b)
        self.hue.set(h)
        self.saturation.set(s)
        self.value.set(v)
        self.hexa.delete(0, "end")
        self.hexa.insert(0, color.upper())
        self.bar.set(h)
        self.square.set_hsv((h, s, v))
        self._update_preview()
Respond to user click on a palette item.