California White rabbits have a special gene that is responsible for their characteristic markings. This gene is temperature-sensitive, meaning it makes melatin in response to cold. The coldest parts of a bun’s body are the tail, feet, nose and ears. Rabbits with this gene gets a lot darker ears, feet, nose and tail in in winter – even indoors rabbits get markedly darker.

I would assume your doe’s dew lap is cold-ish. If a bun has been fat, the dewlap can persist, even though it’s “empty”, meaning the fat in it has been used up as energy but the skin doesn’t have the elasticity to shrink back. (The same penomenon is of course seen in humans that have lost a huge amount of weight – they have to have their surplus skin surgically removed). If the dewlap protrudes much like an Elizabethan collar, that too could mean it gets colder than the rabbit’s core temp. 

If your girl’s chin pouch is just an empty “sack”, it would make sense that it gets a little bit cold. That could turn the Himalayan gene on, especially now in winter.

Here’s some info about the Himalayan gene:

http://chimalaya.org/2011/02/01/genes-that-respond-to-the-environment/

Picture from http://www.nature.com/scitable/topicpage/environmental-influences-on-gene-expression-536

You don’t have anything to worry about. Californians are sort of Thermosensistive coats… their color points: ears, tail, feet, nose and dewlap, will get darker in colder seasons and lighter in the warmer… they will also deepen as she matures, in case you got her while she was still a baby. Rabbits in general are comfortable in temperatures 68-mid 70’s and should not be overly warm nor exposed to drafts. I live in a drafty 100 year old home and I keep the house as comfortable around 68 -73 as possible to avoid my own pain flare ups and I can only say from my own BunBun that she still turns colors with the seasons no matter what temperature the home is. We actually really enjoy watching it bcz she develops stripes in her color points! Quite the fashionista!

Like Muj Mom N Bun says, you don’t need to do anything anbout the temperature. It’s perfectly normal for a California White to change color a bit during the cold season. If the fur looks and feels normal and the skin underneath is fine, it’s just a normal genetic quirk typical of her breed.

If a bun gets no much moisture on their chest or cheeks, you don’t get a gray color fur. You get tangled hair first, then hairloss and red, compromised skin.