As you say pagan folklore is responsible for a lot of Christmas tradition. Certainly when the Romans invaded northern Europe they took local beliefs and incorporated them into the Roman belief system. So when the Roman empire became the Holy Roman empire all the pagan stuff like the Green Man would have been still there. Pagan cultures in northern europe must have needed something to brighten up a cold winter so bringing an evergreen plant into the house must have been widespread.
As for red, from this link
“The Civil War cartoonist Thomas Nast drew Santa Claus for Harper’s Weekly in 1862; Santa was shown as a small elf-like figure who supported the Union. Nast continued to draw Santa for 30 years and along the way changed the color of his coat from tan to the now traditional red. Though some people believe the Coca-Cola Santa wears red because that is the CokeĀ® color, the red suit comes from Nast’s interpretation of St. Nick.”