Rules. As a type-A, I'm usually a fan of rules, except in design. I'm not afraid to mix metals, I don't think small spaces can't have dark color, and don't even get me started on the 'work triangle'. Every situation is different: from the lighting to the layout, every space needs its own plan; its own life. If your designer said, "Sooo... we're going to do white and navy cabinets with gray counters..." So far, you're thinking it's safe. Then, "...The blue on the range is more of an aqua with brass accents, but the rest of the appliances are stainless. The faucets will all be chrome (except in the laundry room which will be brass), but the hardware and lighting will be gold. We're going to have a mix of solid and glass-front cabinets, with some open shelving. We're going to paint your fireplace as an alternative to replacing the ugly stone. Oooo, and we're doing two different wallpapers in different blues and different patterns. It's going to be fabulous." You might be thinking your designer is cuckoo. Then she waves a magic wand and shows you the finished space. You realize why you hired a designer. Especially one with an entire team that likes to break the rules.