- Green: safe, okay
- Yellow: warning
- Red: danger.
So suppose you’re designing a floppy disk drive. It will have a light that flashes whenever data is being written or read. What color should the light be?
In this case one purpose of the light is to tell you when NOT to remove the floppy disk, so yellow is a good idea. Red would convey the wrong message (it’s not dangerous or bad to read the data).
My cell phone has a status light that’s on as long as the phone is on. Why is it red?
What got me thinking about this is I my new USB memory stick that lights up a pretty blue to tell me not to remove it from the computer. (It’s okay to remove it when the light is off.) Why blue? Strictly a cosmetic issue I think; the Swiss standard would suggest red in this case. (Or alternatively, green to show that the memory stick is working okay.)
Next time you see a status light of any kind, have fun deciding what color it should be to conform to the Swiss standard. It’s a pleasant parlor game, and you’ll find lots of tough cases.