(the only thing this room needs is me sitting in it, sipping a dirty martini with extra olives. monochromatic olive green room complete.)
The concept is simple: pick one color and decorate a room with only it and shades of it. This is actually not oversimplified and results in a gorgeous, unified looking space, evidenced by the images above and below. Here are a few guidelines to follow that will help you to achieve a room with an elegant cohesiveness and modern feel. The short version is make sure to vary textures; this is key when decorating with one color. An example is the leaf green foyer below that uses glass, wood, and fabric, all in varying shades of green.
(LOVE the green glass. note that the shades used are more note-in-a-bottle and less Heineken)
(notice the mismatched chairs still look formal because there are only 2 different sets mixed together and they are arranged symmetrically)
(I have a well documented love of the color gray in decorating, I am now dying to add silver/mirrors to my gray space.)
Photo References (in order of appearance):
(photos 2 + 3) http://www.porterhousedesigns.com/colorsizzle/