Off White color paint first appeared on Canon lenses in 1976 on the FD 600mm f/4.5 and FD 800mm f/5.6 lens. Why are some Canon 'L' lens painted Off White while others are black in color?
According to Canon, the technical reason is : Heat Reduction. Lenses contain many glass elements. These optics expand with heat, over time. Large lenses contain more and larger elements and heat expansion can bring a lens close to the limits of its design tolerances. A lighter surface reflects sunlight and helps to keep the lens cooler. In shorter lenses, the amount of glass expansion is smaller and usually do not cause a problem.
For outdoor sports and wildlife photography, being exposed to the harsh rays of the sun for long periods of time can cause damaging heat expansion. The lighter color of the lens helps to reduce overheating and protects the internal optics and image sharpness of the glass.
A number of Canon's black colored lenses also have a heat-related feature. A lens that can focus past its infinity mark allows the glass to expand when shooting under hot conditions.
Over the years, Nikon has resisted going to the lighter color but Sony has started manufacturing their high end glass with 'white' paint as well. I have been on numerous wildlife photo shoots in India and Africa and the conditions are quite harsh and hot. You can see my equipment bag and works on MichaelDanielHo.com