Cool Blues and Silver Stars are actually optically coated to remove the yellow wavelength. They also use a different gas in the casing. They produce a whiter wavelength, and brighter in the more visible spectrum. They work better at night on wet roads for that reason. I've run both, and the Silver Stars are superior, but even the cool blues are vastly better than a stock lamp.
(For those of you who don't know, I'm a Laser Technician....optical coatings and wavelengths of light are my forte......to lend a little credence to my ranting).
So you can gain the same benefit of the HID to a slightly smaller degree for much less money.