Form
does follow function. I'm not sure whether you really don't understand why some of Canon's lenses are white, or if you're just trolling?
Either way, for those that don't know:
Canon uses Fluorite in their pro lenses. AFAIK, Nikon doesn't currently.
http://en.wikipedia.org/wiki/Fluorite
"...
Fluorite is used instead of glass in some high performance telescopes and camera lens elements...
Fluorite has a very low dispersion so light diffraction is far less than ordinary glass, and in telescopes it allows crisp images of astronomical objects even at high power..."
Unfortunately, Fluorite is temperature sensitive. Actually, all lens elements are affected by temperature in some manner. Painting the body of the lens white keeps the temperature inside the lens barrel cooler.
Nikon uses 'ED' glass in their lenses. I've read that 'ED' glass is a composite of several materials, including Fluorite. I've also read reports that the focus shifts with temperature on these Nikon lenses and that they are designed to focus past infinity to cope with this.
In short, if you're a fashion conscious photographer, buy Nikon and get color coordinated lenses. Otherwise, buy Canon and get (theoretically) more consistent lens performance.