I am not sure if your scars are red but I think its probably something to do with blood vessels. They are probably dilating in the cold cuasing the darker color. Not positive but I remember I have similar experiences. Maybe something to help with redness (blood vessels) can help, like IPL laser or Vbeam laser.
I know this is an old post, but does anyone else have an explanation for this? Or maybe even a way to help keep scars from getting darker whenever you get cold? I have been using a topical foam that has helped fade the scars on my arms and they look fairly light when I'm warm, but whenever I'm cold, it's as if every single scar has reappeared.
Is it just your scars that get darker or have you noticed that your skin tone in general seems to go a bit darker or blueish? I know when my fingers are cold they go a blue-grey colour. I have a thin scar on the back of my hand which magically reappears when my hand is cold. I can't say I've had my face change colour when it's cold though. I have no circulation problems and am otherwise healthy.
I believe that this is known as Raynaud's disease. Your blood vessels narrow when you're cold and thus your blood cannot get to the skin's surface as easily, causing discolouration. The scars will show up as a darker colour as there is even less blood flow in them to begin with. Sadly, the only thing you can really do is... keep warm. Using concealer may help to lighten your scars when it's cold. Maybe this is worth a trip to the doctor?
Oh, as a side note, have you looked into treatments that improve blood circulation? The Dermaroller is known to increase blood flow to scarred areas, thus making them an even colour with the rest of your skin, in time. It can be done anywhere on the body. Maybe there are some before/after pictures of this and/or others who have tried this method?
Good luck!