I found out just now that blue is the only color in the state of Washington that is specifically prohibited.

WAC 352-60-060

(1) The requirements in this section shall be complied with in all weathers. The requirements concerning lights shall be complied with from sunset to sunrise, and during such times no other lights shall be exhibited, except such lights as cannot be mistaken for lights specified by the United States Coast Guard, or do not impair their visibility or distinctive character, or interfere with the keeping of a proper look-out.
Navigation lights and shapes.


(13) Law enforcement vessels - Inland waters.

(a) Law enforcement vessels may display a flashing blue light when engaged in direct law enforcement or public safety activities. This light must be located so that it does not interfere with the visibility of the vessel's navigation lights.

(b) The blue light described in this subsection may only be displayed by law enforcement vessels of the United States, Washington, and its political subdivisions. The use of blue lights by other vessels is prohibited.

(14) Public safety activities - Inland waters.

(a) Vessels engaged in government sanctioned public safety activities, and commercial vessels performing similar functions, may display an alternately flashing red and yellow light signal. This identification light signal must be located so that it does not interfere with the visibility of the vessel's navigation lights. The identification light signal may be used only as an identification signal and conveys no special privilege. Vessels using the identification light signal during public safety activities must abide by the rules found in WAC 352-60-060, 352-60-065, 352-60-066, and 352-60-070, and must not presume that the light or the exigency gives them precedence or right of way.

RCW 79A.60.120
Tampering with vessel lights or signals—Exhibiting false lights or signals—Penalty.
An operator or owner who endangers a vessel, or the persons on board the vessel, by showing, masking, extinguishing, altering, or removing any light or signal or by exhibiting any false light or signal, is guilty of a misdemeanor, punishable as provided in RCW 9.92.030.