Red = "I need help urgently!"
Yellow = "I'm OK right now."
Green = "I'm really confident in what I am doing and can help others."
The green students could also be encouraged to be the "class experts" who can be helping others who have their red cups out! This is a fantastic tool in promoting independence in your students and saves having lots of hands up and calling out in class. It also allows you to monitor the progress of your students in one quick glance and helps those "invisible students" who might not be as vocal in asking for help.
Just be sure to explain that these are not for playing cup stacking though, as it is tempting...!