Fair warning: This answer is a real stretch, but if you squint hard I think it'll work for you.
Borrowing From Others
All of the traits from your question do exist in animals already; however, they are spread out over a few different species. Between three animals, I think we can amalgamate an Elf that suits your needs, mostly.
The puffer fish is well known for its ability to expand on demand. Generally, when a puffer fish is scared, it pushes water into its stomach, which unfolds into the cavities of its body until it's a tight, round ball of spines. The skin of the fish is made up of collagen fibers, which allow for an expansion of up to 40%. Also, puffer fish are known to suck in air as well as water to the same affect. All of this is useful for us, because we want a skin structure that can expend and contract at will.
Imagine a humanoid with tiny bags of collagen-constructed flesh all over its body that have biological "ducting" of some kind running to each one. At will, your Elf could inflate some or all of these little bags with air or other fluid to whatever degree you desire. I would suggest air here, for several reasons: 1) Air has insulating qualities, which might address Adrian's comment about temperature, since an Elf wrapped in pockets of air would be able to stay warm, similar to wearing a coat. 2) Moving that much blood would be very taxing, and it is not as available as air 3)Water would be heavy to carry that much around, and also less available than air.
Air provides a bonus that it's feasible your Elves could float really well in water, and cushion a long fall.
Chameleons are covered in cells called chromatophores that respond to signals from the brain and bloodstream. These cells respond by shifting color. They typically only have four colors to work with - yellow, red, blue, and brown - but, like LEDs, if you have those colors display together in different mixes you'll get other colors, like red, orange, etc.
How this applies to the Elf is obvious. What is less obvious is that we have the expanding skin to deal with as well, and it wouldn't really be feasible to have the chromatophores on top of the skin that expands. Rather, it would be biologically easier for the color cells to be in the tiny spaces between sacs. You did specify that they would only generate colors when the temperature is warm, which solves the problem. When the sacs aren't inflated, you could clearly see the colors, but when they are it would be much harder if not impossible. Or you could have the chromatophores on the sacs, but this way you have the choice if you need an explanation for how they behave.
Humans possess the ability to manipulate their hairs at will, make them stand up and lay down, via tiny muscles that contract at each follicle. Replace the hairs with your sacs and you have a method to control the position of those sacs relative to the skin with less hand waving than otherwise. The only downside (or upside, depending on your perspective) is that your Elves wouldn't have hair where they have sacs. This makes sense in the context that the sacs, when filled with air, provide insulation, so they don't need hair from an adaptation standpoint.
What I am not touching on here is process of a humanoid adapting these structures. Or the pockets. Pockets can just naturally exist without issue, like with a Kangaroo, but creating them at will means skin ripping itself apart and the closing up later, and I just can't rectify that biologically or scientifically within a useful time frame. Please enjoy these thoughts, and good luck building!