Measure your ceiling height before you measure anything else, because every tree looks shorter in the shop than it does crammed into a corner with a star on top scraping the paint. Leave at least a foot of clearance. I learned this the hard way with a tree that looked perfectly reasonable in a warehouse with twenty foot ceilings and then had to have its top third removed with kitchen scissors in my actual living room.
A tree is basically the biggest, most temporary piece of color you'll bring into a room all year, so think about what it's standing next to. Deep green against a red brick fireplace is gorgeous and a little bit obvious, in the best way. Against a navy or charcoal wall it goes almost jewel toned, especially with warm white lights, which is the only light temperature I'll defend to the death. If your living room is already doing a lot, busy rug, patterned couch, art everywhere like mine, let the tree be the calm one and put your personality into the ornaments instead. My mother's living room is beige with a beige tree and I genuinely don't understand how she sleeps at night, but even she admits the gold ornaments against that much neutral look rich instead of boring. That's the lesson. Contrast works both directions.
On delivery day or in the shop, check the trunk is actually straight when the tree is upright, not just leaning against a wall where gravity's doing you a favor. Fluff the branches before you decide you hate it, real and good fake trees both need five minutes of you pulling branches away from the trunk or it reads flat and sad. Check the base is sized for the trunk you've got, because a wobbly tree next to a toddler or a cat is a holiday card nobody wants to send. And look at it in actual room light, not shop fluorescents, because a tree that looks rich green under warehouse lights can go strangely blue or artificial looking once it's home.
Where it clashes is with a room that's already covered in cool toned, icy decor, silver and white everything, because a warm green tree next to a cold palette fights instead of glows. And it clashes, hard, with a small apartment living room where the tree is the only furniture that fits if you squint, leaving no path to the couch for six weeks.
The one mistake to avoid is buying on height alone and ignoring width. A tree that's narrow enough to walk past is worth more than two extra feet of tree you have to climb over every single day until January.