[weeny]

The idea is that the yellow phosphor converts a short wavelength blue light from the LED into a longer wavelength yellowish light, called down-converting.

This is a special physical property of the phosphor material - not simply a function of its color.

Normally this yellow phosphor material is applied directly to the LED die - but they figured they would put it on a plastic shell outside the LEDs. It doesn't actually make any kind of performance improvement except that the light is more evenly distributed.

If you tried to paint the shell with the phosphor - you would most likely have blue light shining through it - that's why they need so much technology to accurately dope the plastic phosphor, so that the light output is an even and consistent color.