Bearded Dragon suden colour changes?

[jessieJEALOUSY]

I may seem like a total un-educated idiot, but i was curious as to why my bearded dragons under-go such contrasting colour changes? I went into check on them earlier and noticed one was very bright, i picked him up, assuming he was the largest one i own as he tends to always be a bright yellow, but noticed he was actually my smallest dragon, who is generally a very dark grey.
I returned, maybe 20 mins later, and he was back to his dark grey, i did double check when he was yellow that he was actually batman (my smallest) i checked his patters and looked at my other dragons.
It just seemed so strange; might have to change his name to Dr. Octo or something now =\

Incase the question was lost in my ramble.
Why do, or How do, Bearded dragons under-go these changes?

 

[ozianimals]

From what I believe it has something to do with their moods.
I don't know much about lizards/dragons, but remember something about it on a nature programme.

 

[naledge]

My beardie usually changes his colour depending on the temperature. He's yellow/orange when it's warm and grey when it's cold.

 

[drunken]

most of lizard has this skills, make themselves dark to get warm~