Walk into an office supply store, and you'll see highlighters in almost every color imaginable. So why is yellow still the most popular highlighter color? Because, even though yellow might not be your favorite color, your eyes naturally see the color yellow much more easily than, say, blue or red. The reasons for this have to do with the wavelengths of different colors. Think of a rainbow, the range of colors of visible light. It goes from red, the color with the longest wavelength, through orange, yellow, green, blue, and indigo to violet, the color with the shortest wavelength. Yellow and green are right in the middle of the spectrum's wavelengths. Our eyes are most sensitive to yellow and green, so they're the easiest colors for us to see, even when we're not looking directly at them. And even people with red-green color blindness can still see yellow, that’s why it's the most popular color for highlighters. So, if we notice yellow and green more quickly than other colors, then why are stop signs and most emergency vehicles painted red? That's a good question! The choice of red has more to do with tradition than visibility. In fact, many communities have started to paint their fire equipment and ambulances a very bright yellowish-green, which helps motorists to see them coming, and get out of their way more quickly.