Tie red and green string together. Put it in a can of some sort (used and washed out plastic frosting can, for example). Pull out the string as you sing. Green - sing! Yellow - hum! Red - think the words in your head!
HERE is the idea on Music For Primary's Blog. Hers is cuter than mine. :)
**The problem with this is that you can't see the string in the back. In fact, you can barely see it on the front row. Solution: Hold up [or have a child do it] red, green and yellow paper (in sheet protectors) that is the same color as the string coming out so everyone in the room can see. (As seen in this post, Stop & Go Review)