Why is Green and Red the colors of Christmas?

Holly berries. The holly tree stays green throughout the winter and fruits around now. While not great eating it provided calories in the darkest part of the year.

Humans can't really eat those. Since there are shitloads in the forest around my house, I looked about using them for forage. Even a couple will irritate your gut.

Otherwise, your explanation feels right. The hollow bushes are most of the color in the woods around here.
- From wiki
  
  Holly berries can cause vomiting and diarrhea. They are especially dangerous in cases involving accidental consumption by children attracted to the bright red berries.[32] Ingestion of over 20 berries may be fatal to children.[31][32]
  
  Holly leaves, if eaten, might cause diarrhea, nausea, vomiting, and stomach and intestinal problems.[32]
  
  Holly plants might be toxic to pets and livestock.[33]
  
  Holly berries are fermented and distilled to produce an eau de vie.[35][36]
  
  (Fruit brandy).
- Ive been told they can be fermented and turned into wine.

The Xmas tree probably originates from the Nordic" Tree of life ". I remember reading it was used to celebrate life and the return of light during the dark cold winters in the north

To celebrate and remember the return of the sun in the dark Winter, lights were used to chase away the dark; green as a symbol of returning life, like vegetation and plants, and red probably symbolises fruit and harvest.

There are several versions and interpretations, so it probably depends on the country or region the tree was used, and in which century.

Santa Claus ( yes, created in USA by Coca-Cola), was /is originally Saint Nicolaus, which in turn was probably merged with Wodan This is the same God as in Wednes-day ( was originally Wodan's day, a Nordic God; just like Thursday, was originally Thor's day ).

Many old Germanic/Nordic religions ( they were very similar) were integrated with Christian symbolism and rituals by the Roman Catholic Church.

Check this link for more general info.

Also because of the ancient Dark versus Light theme, there is definitely an association with the Wild Hunt

In the Norse myths, the original leader of the hunt was the god Odin, known in Germanic myth as Wodan. Odin rode his eight-legged horse, called Sleipnir. His company of hunters were the Valkyries and the dead warriors who resided with him in Valhalla.

The gifts placed in the Christmas trees have also an association as an offering / gift " to forefathers who fell as warriors, and or offering to "the wild hunt" and it's phantoms, as a sort of guarantee you wouldn't invoke their wrath and be taken by them to the phantom realm.

Saint Nicolaus in it's older manifestation had a stronger dark and light side. His demons where probably black or dark, like we see in Austria, Kruppe and Black Pete in the Netherlands/ Flanders region.

This is a super interesting comment, thanks!

As for why Coca Cola made their Santa red, I guess the Scandinavian influence is also worth keeping in mind.

The Norwegian version of Santa is known as Julenissen - literally the Yule nisse - with the continental European stories of Saint Nicholas having been merged with our own traditional nisse (Scandie gnome) living in the barn (fjøsnisse). Nisses, like gnomes, traditionally wear red hats.

Where I celebrated Christmas on Tuesday, we were visited by a version of the fjøsnisse rather than coca cola santa. He told the kids they are coordinating with the Christmas nisse, as it gets busy on the north pole (there is, of course, only one Christmas nisse, but a bunch of fjøsnisser). But of course a knitted red hat was still in place. 🎅
Back 1700 years ago, when St. Nicholas lived, red was an expensive color, reserved for nobility and higher clerics. As Nicholas of Myra was a bishop, he is traditionally depicted in red bishop robes, which advertizing later turned into that red suit worn by fat Santa.

Our association between Christmas and those colors comes in part from:

old traditions
CocaCola marketing

Basically, those colors have always been associated with this time of year and festivities of similar nature ever since ancient civilizations. And in modern times, thanks to the fizzy drink marketing campaign and the influence of the US on the rest of the world, the red and green image of Santa Claus and other decorations they became the default colors for Christmas worldwide.

Here's an article about it. The Wikipedia page also has more info and sources on symbols of the holiday :)

Christmas has always been a celebration of mid winter ( more specifically the winter solstice in the northern hemisphere,) as far back as records go there has always been some variety of taking a evergreen tree as a symbol of the return of the sun. Red might be part of the evergreens berries (holly), but mostly it's because coca cola advertised with a picture of St Nicklaus dressed in their corporate colours, instead of the traditional green.