Safewords are the epitome of BDSM's principles of play being safe, sane and consensual.
A person's safeword can be very personal and can say a lot about the person. However, there is one universal safeword that stands out: red.
Yellow is for caution.
Green is for go.
Red, yellow, and green make up the colors for a stoplight in the U.S. I'm told that stoplights range in color in other parts of the world. You can verify this in your comments, of course. :-)