Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. Red means stop. Red means danger. Red means passion. The color conjures up a whole range of emotions and associations. It inspired an entire Taylor Swift album. And yet if someone asked you to describe what red actually looks like, without pointing at something red, you’d hit a wall almost immediately.  So why is it that a color so evocative and distinctive as red (or any color, for that matter) still manages to elude our attempts to nail it down with words?  If you just now said, “It’s because color doesn’t exist,” well played!  If you’re like me and your face just turned an indescribable shade of red, welcome to the club.  “There is no color in the world,” says American neuroscientist Christof Koch . “There are photons of a particular wavelength emitted by the sun that strike an object, and then get reflected into the eye of the viewer.…