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.

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.

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 :)

Red for Santa, green for coniferous trees, white for snow

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.

Snot and blood

Green for conifers.

Red for the mushrooms that pop up at their base

https://www.ffungi.org/blog/the-influence-of-hallucinogenic-mushrooms-on-christmas

I always thought it was for mistletoe which has green leaves and red berries. But maybe that’s something that came after red and green were established as Christmas’ colors?

Red because you are tired and sweaty from all that xmas shopping. Green because all the oligarchs have all your money now.

Red: the blood of angry men.

Green: the blood reptilian healthcare CEO’s.