My DCs are still quite young. The older one was 2 when their room was decorated. I chose the decor in line with what he liked at the time, and what was practical long term. It's plain, so things like bedding and pictures could easily be updated should the DCs express a wish about it. The decor is still good, so when that needs renewing they will be an appropriate age to have input into what we do. I probably would veto black as I've had unpleasant experience involving painting over black in the past 
The playroom is an area that's frequently left to be (then has a blitz to rediscover the floor). The lounge is more of an adult space. Toys are allowed in, but less of them and removed frequently. I'm failing to maintain my bedroom and bed as adult space as it's subject to nocturnal invasion!
They choose their clothes, but I manage them- washing and filtering out the out grown stuff. Their toys are theirs, although DS2 latched on to one of my special teddies from my childhood. She is his to love, but mine to keep if he doesn't love her any more.
We talk about sharing a lot. Both DCs are boys of close age gap and similar interests, so sharing works in general. Special things like drawings, particular cuddly toy must be respected as an individual's. Currently technology like the tablet and laptop are shared. This helps with monitoring too.
I think we fall very much into mine and DH's house, but our family home. The input of the DCs will change as they get older.