The court brief I promised:
http://www.mediafire.com/view/30onoy9ecordwuv/UnmarkedCarCourtBrief2014.pdf
Also, some quotes from the Washington State Patrol Regulations Manual:
Page 438-439:
SECTION 11: MARKING AND EQUIPMENT
17.11.010 PATROL VEHICLE MARKING AND EQUIPMENT (CALEA 41.3.1,
41.3.2, 43.1.5, 61.1.6, 70.4.1)
3. Troopers
a. Vehicles issued to traffic officers shall be white.
b. The vehicle shall be marked on the front doors with a black and
white Washington State Patrol shield and dark blue lightning
bolt. Decals with the words "State Patrol" shall be affixed to
both front doors.
c. License plates shall include the officer's personnel number.
d. The vehicle shall be equipped with a roof-mounted light bar,
blue spot light, roll bar, jail partition, push bars, siren, radio,
and standard trunk equipment.
Page 442:
SECTION 13: CONFIDENTIAL LICENSE PLATES
17.13.010 CONFIDENTIAL LICENSE PLATES (CALEA 43.1.5, 61.1.6)
I. POLICY
A. Use of Confidential License Plates
1. Confidential license plates shall only be utilized on vehicles within
the department for confidential, investigative, or undercover work;
or when necessary for the personal security of any officer or public
employee.