they use the beard for lots of things...their beard will change colors sometimes because of nothing, or if they are around another dragon, or if they are angry...they just do that...hence the name bearded dragon
i think it's correct to say the beard is used for different things, beardies are very expressive. the beard may be black and they may sometimes gape if in fear, or perhaps a dragon might have the beard out to show dominance (like when out of the enclosure and exploring new territories). mine might show me a black beard if he's just annoyed by something. so i think there are a variety of scenarios, and once you've been around your pet for awhile it will become more clear what they're trying to say. i have a 2.5 year old male beardie and he's full of personality and quite fun.