What is a "sound signature"?

Q: What do you mean when you say "headphones have specific sound signatures"?

A: Different headphones have different signatures. For example the Beyer DT880 or Senn HD600 are more "neutral" sound profile, i.e. they are free from coloration,

Coloration refers to deviations from neutrality, for example the Philips Fidelio X1 is very colored. Coloration also often adds a "fun" element to listening.

Some headphones are intrinsically dark -- the term signifies a warm + rich sound profile. Technically, dark headphones have a frequency response which is skewed clockwise across the entire range, so that as the frequency increases, the output diminishes. A good example of this would be the HD650.

Other headphones are very bright, their frequency response is skewed counter-clockwise across the entire range, so that as the frequency increases, the output increases. A good example of this would be the Grado SR60i or e (or pretty much any Grado).