Please specify what you mean by monochrome...
You can employ
xrandr scripts clickable on a panel. I'm using for example following script
xrandr --output VGA-0 --auto --right-of LVDS-0 to enable VGA output on the right from my notebook. Please specify what you mean by monochrome... by xrandr you can simply switch graphics mode of any graphics output, so it could be on one clik in the menu panel. Read
xrandr --help on how to switch it to different monitors/modes
You can use
xrandr to tune colors... can suit your needs. watch for brightness and gamma settings
Update: you can use
xrandr to change saturation as well by the set attribute, example:
xrandr --output TV --set Saturation 0-100. But the device and the driver should support it... otherwise you will get BadName error. I can imagine, that you will change it trough the color profile of your display. Solution very much depends on what you have.
Monochrome icon/colour theme
You can switch monochrome theme on the fly in System settings - windows manager by choosing a monochrome theme. you can run settings from command line by
xfwm4-settings see the man page for details: http://swoolley.org/man.cgi/1/xfwm how to switch it from the command line. Again you can make it part of panel, so it could be made on one click.