Shimano cleats come in 3 different colours these days. The latest version is blue, which hasn't been around for long but is definitely worth considering.

THE RED CLEATS
The SM-SH10 has been developed for the trained cyclist who knows how he wants his cleats set up. This can be done through either a proper "cleat fitting" or simply through experience. Injuries, predominantly to the knees, are easily sustained when your cleats aren't set up 100% right. The red Shimano cleats are the most efficient as the shoes will be fully locked down into the pedal. This means no energy is wasted by repositioning the foot after every pedal stroke. This usually happens as your foot is prone to twisting marginally during the dead zone in the pedal stroke.

THE YELLOW CLEATS
The SM-SH11 is included with every set of pedals. These are designed for the average rider: they allow for some movement of both the heel and toes when engaged. The shoe essentially pivots around the heart of the cleat. In short: these cleats are the least efficient, as they were designed to prevent injuries from poorly fitted cleats, by offering some play in all directions.

THE BLUE CLEATS
The SM-SH12 is a recent invention by Shimano, and combines the properties of the red and yellow cleats. The pros asked for a cleat with which the heel could still move a little, but it still had to feel as "locked in" as with the red ones. The round blue edge secures the shoe like with the red system, but still allows for 1 degree of play at your heel. In short: nice and tightly locked in, but easier on the knees because of the minimal amount of play.