What kind of Concepts can be used for interior designers?

I don't know a lot about interior design, but I do know that trends play a big part, like shag carpenting of the 70's-haha! Color is important dependent on what you want the room to be. A bedroom should be calming and soothing while a play room should be bright and full of motion. You also want to use focal points. If a fireplace is a focal point then you will want the arrangements you make to lead into that. You can use tricks, too, like making a tall room appear shorter by dividing the wall with a chair rail or making small areas look bigger by using smaller pieces for accents and lighter colors.