I bought a package of each color of cups at Party City a few years ago. I'm still using the same cups, so they've held up well.
Each group gets a stack of three cups. Red on bottom. Yellow in the middle. Green on top. At least that's the way they start the activity.
As groups work together on the assigned task, they change which cup is on top of the stack to signal what they need from me.
If the green cup is showing, I know they are working well. If the yellow cup is showing, I know that group has a question, but they can keep working until I get to them. If the red cup is showing, I know that the group needs urgent help and feels they can't move forward without me.
Here's a picture of the cups in action:
As you can see, this group is working well. If you're interested, I blogged on the activity they are working on here.
To jog students' memory about what the cups stand for, I made a set of posters to display. I printed the meaning of each color of cup on that color of paper. I didn't know what color to make the title, so I just made it blue. That was a completely arbitrary choice.
Green Cup Poster:
Yellow Cup Poster:
Red Cup Poster:
If you'd like to download the files for these posters, I've uploaded them here as an editable Publisher file and a non-editable PDF file. If you go the Publisher route, you'll need to also download these free fonts or substitute your own favorites: HVD Comic Serif Pro and Ostrich Sans Heavy. If you don't want to fuss with fonts, just choose the PDF version.