Do you remember "mood rings"? Back in the day - if I recall, a year or two before "pet rocks," they were a fad. They would supposedly change color to indicate what sort of mood you were in. I believe the technology was based on skin surface temperature - definitely low-tech.
Technology has marched on.
Arlene Ducao is a research assistant at the MIT "Media Lab Information Ecology Group." She's also a bike commuter. And she has invented the "MindRider" - a bike helmet equipped with an EEG sensor that displays your stress level.
According to the article, it uses "the visual vocabulary of traffic lights. Green lights indicate a focused, active mental state; red lights indicate drowsiness, anxiety and other states not conducive to operating a bike; and flashing red lights indicate panic."
Ms. Ducao sees this as a possible way to improve safety for cyclists and motorists. "One way of doing that is improving communication between cyclists and drivers." Her helmet is just a prototype, but she wants to improve the ergonomics, and also a tracking system (probably a phone "app," huh?), so you can figure out which routes are high-stress and which aren't so much.
So - what color would your lights be? I like to think that in most of my bike-riding circumstances, I'd be solid green.