I know this sin't about the "legal warning light colors" but it is an interesting little tid bit I found...
(625 ILCS 5/12-207) (from Ch. 95 1/2, par. 12-207)
Sec. 12-207. Spot lamps and auxiliary driving lamps.
(a) Any motor vehicle may be equipped with not to exceed one spot lamp and every lighted spot lamp shall be so aimed and used upon approaching another vehicle that no part of the high-intensity portion of the beam will be directed to the left of the prolongation of the extreme left side of the vehicle nor more than 100 feet ahead of the vehicle.
(b) Any motor vehicle may be equipped with not to exceed three auxiliary driving lamps mounted on the front at a height not less than 12 inches nor more than 42 inches above the level surface upon which the vehicle stands.
(c) The restrictions of subsections 12-207 (a) and 12-207 (b) of this Act shall not apply to authorized emergency vehicles or equipment used for snow and ice removal operations if owned or operated by or for any governmental body.
(d) The minimum and maximum height restrictions prescribed in subsection (b) of Section 12-207 shall not apply to privately owned motor vehicles on which a snow plow is mounted, while in transit between or during snow and ice removal operations. This exemption shall apply only during the period from November 15 through April 1, and only when the snow plow blade, commonly referred to as a "moldboard", is properly and securely affixed to the front of the motor vehicle.
(Source: P.A. 85-1010.)