Something you guys have to know about lotr dragons...
Unlike msot fantasy dragons that comes with green, blue, red, purple type of dragons, lotr dragons are in most grey/whitish and black and they do change colour of a sort like chameleons, not to mix with the enviroment, but they do change colour of a sort depending on their mood.
As you see on the first movie, smaug on his original self hes grey but when a dragon is sleeping he reverts to like bears.
The concept is literally like this:
"Hibernation (from Latin: hībernus, of winter) is a suppressed metabolic state that falls under the umbrella-term of torpor or dormancy.
It is but one of many forms of metabolic suppression. Hibernation is a widespread and common survival strategy expressed under
the threat of a metabolic energy crisis. Conversely and considering that some animals exist in this state for the greater part of the year,
it could just as easily be considered the default metabolic state and up-regulation of metabolism merely a response
to the availability of energy. As the name suggests, hibernation has a seasonal connotation that points
to triggering by the threat of lowered ambient temperatures, famine, and darkness, e.g., autumn and winter;
hibernation during the summer months is called aestivation.
Often associated with lowered ambient temperatures, the central function of hibernation
is to ensure survival by conserving energy in anticipation of a period of food shortage.
The down-regulation of metabolism lowers metabolic needs and enhances tolerance of cold.
To achieve this energy saving state, endotherms entrain a series of energy-conserving physiologic response,
like peripheral vasoconstriction, bradycardia, lowered respiratory needs, core-body cooling, etc.
This has the concurrent effect of lowering whole-body metabolic rate, by effectively isolating and letting cool peripheral tissues
from central and more `vital´ tissues. In this stage, core (visceral) temperature may remains stable;
it remains undetermined whether visceral temperature reflects brain temperature.
As explained above hibernation in this case of dragons they just keep body heat low except the vital parts,
what happen to a human when its dead? dont he turn out purple and even more whitish with the cold affecting those parts?
We can assimilate dragon´s hibernation to something like that, the cells stop running to maintain all body and just keep
the vitals turning most of the body colour less healthy (white or darker.. or purple like dead thing... )
And same happens to smaug, hes grey as standart colour and when blood stop flowing through his body to let him hibernate
its a normal responce to get the body left to care for heath = skin of grey turns darkeropal kinda like polished blackbluish..
Now why you see the dragon red you wonder?
Like I mentioned above, dragons act like chameleons what matters to his mood, its pretty nature like when
some animal like snake or insect looks red to warn "its dangerous". Same Applies to Smaug, the red coloured skin
is something like BBC documentary about stegossaurus flowing blood into his plates to turn red as a sign of warning to the predator hes armed and dangerous.
watch this movie and you´ll see... keep attencion to the plating turning red and the documentary explanation.
http://www.youtube.com/watch?v=eUdcrYvhJr8JUst in this case red dragon means, and a very very very angry dragon... which reveals the lacks of patience
he had about bilbo talking and when he got utterly angry at the lake men...
In the end you turn up just like this...
Also look the detail of the eyes... more blood flowing means more bright eyes, he turns from a yellow/golden eyes from sleeping into bright orange fiery eyes when hes angry..