I did that in my guest room - no problem at all. The carpet is pretty low pile and the rug is a fairly thick sculpted type oriental. I have no pad under the rug. I also have a very short, dense wool oriental rug on the carpet in my living room. That also works fine. I have all these rugs on carpet because I moved from a house with wood floors to one with more carpet. I hope to get rid of the carpet someday when I feel like moving everything again. So then I will need the rugs and in the meantime I can't stand the plain beige carpet. In another room I have a thick wool runner that covers the width of carpet from the deck door to the wood floor in the dining area. It has no furniture on it that holds it in place so it ripples a bit. I need to find a way to keep it flat.

Nope, not odd. Perfectly normal to repeat colors throughout the home. You can always add character to each room with furniture, towels, bedding, art, etc.

should it? No. can it? yes. you can have lighter carpet too. depends on how dark the walls are going to be. if you're doing a light gray, then it stands to reason that the gray carpet will probably be a darker shade, or else you'd have nearly white carpet. It all depends on what you want and what other furniture is going in the room

Picture shown is before painting. I don't have a pic of the master bedroom right now but it's black and dark brown bed with tray ceiling. I was considering Passive for the room but I'm not sure it will contrast enough with Snowbound.