With the roll out of the new iMac in 1998, Apple began to use this monochromatic logo which basically has the same contours as the original rainbow silhouette logo of 1976. No specific color was prescribed throughout Apple's software and hardware line. In the first iMac and G3, G4 products the logo was applied transparently.
Now the Apple logo's shape is one of the most recognized brand symbols in the world. Just by only adding this simple shape to all Apple products and retail stores it identifies them as being Apple; the name 'Apple' is usually not even present anymore.