Taken from Illinois Compiled Statutes
http://www.ilga.gov/legislation/ilcs/fulltext.asp?DocName=062500050K12-215
For Amber lights in IL, as Jesse has posted:
Vehicles of public utilities, municipalities, or
other construction, maintenance or automotive service vehicles except that such lights shall be lighted only as a means for indicating the presence of a vehicular traffic hazard requiring unusual care in approaching, overtaking or passing while such vehicles are engaged in maintenance, service or construction on a highway;
It does not say that an individually owned vehicle can have amber lights for spotting purposes.
For Illinois EMA volunteers:
(c‑2) In addition to the blue oscillating, rotating, or flashing lights permitted under subsection (c), and notwithstanding subsection (a), a vehicle operated by a paid or unpaid member of a local or county emergency management services agency as defined in the Illinois Emergency Management Agency Act, may be equipped with white oscillating, rotating, or flashing lights to be used in combination with blue oscillating, rotating, or flashing lights, if authorization by local authorities is in writing and carried in the vehicle.
Our EMA director in Tazewell county told us that we can't have amber lights in our county on our own vehicles. We can have blue or white/blue lights only with a letter signed by him (as stated in the IL Statutes).
There are differences depending on what state you are in regarding the use of amber lights.