Hi,
Actually Most White LEDs are actually Blue LEDs with a certain altreration so that they appear white. They are expensive because blue LEDs are expensive because of the type of dye used. The type of dye is what creates the color.
So, although you can create white light by using Red green and blue LEDs simultaneosly, most do not go to this trouble.
-Nick

__________________
1988 Jeep Cherokee w/ 410,000+ miles. Now a
100% Electric Vehicle!
NO Gas, NO Emissions, NO Problem! --
http://www.DriveEV.com/