The first Apple Computer Company logo pictured an illustration of Issac Newton sitting under a tree right. Legend has it that an apple falling on his head inspired him to discover the laws of gravity and the basic laws of physics. Much of Apple's business plan involves "enlightenment" and "style," a trend that Apple has retained for as long as Steve Job's control.
Because the Newton Logo doesn't reinforce the idea of personal computers, and it was costly to print, the technicolour apple logo was instated and kept. Also, the coloured apple logo follows design trends of the late 80's.
When Steve Jobs returned to Apple in 1998 the first thing to change was the logo. The current logo was created to look slick and futuristic. It is also easy to recolour for personalization and identification of OSX versions.