An Important Principle of Design - Contrast

I think contrast is one of the most important elements of design. For example, if you have a neutral color scheme, then you should have contrast in materials: smooth and rough surfaces, light and dark color values.  If you have a strong contrast in color - you have by that fact created a good contrast.

Strong Color Contrast

Smooth & Rough Textures: Rock Back splash with Granite Counters

Contrast can also be achieved through "style" a mixture of modern and more traditional.  A contrast in style can be done through furnishings and or accessories.  Too much contrast or variety as it's sometimes called can create confusion and dissonant.  Mastering contrast can be subtle or bold.