As a historical note, there used to be a very obscure section in the California Administrative Code (now obsolete and totally replaced by the CCR in circa 1988) that REQUIRED that any police vehicle in the state used for traffic enforcement had to be clearly marked as such and have its doors painted white.
I was in the business for nearly 10 years before I saw this deployed on a collegue who was assigned an unmarked car to use on a OTS DUI grant. He had hooked a DUI and testified under cross about the car not being marked and was completely Dark blue w/ a slick top.
Crafty defense attorney rolled this one out and the DA dumped the case on the spot after a minute or two of hand wringing while reading this most obscure of all sections.
Alas, I can find no vestage of this section in the new CCR.
However, as inferential support I offer the lack of agencies involved in traffic enforcement operating without at least the front two doors of thier cars painted white.
Even in the contrasting days of Oaklands Tan & Whites, Berkeley's Pink and Whites (!)(Chevy Citations too as I recall), Vallejo PD's Orange and Whites and SFPD's light blue and whites (dodge diplomats?). Even the Bay Area Air Qaulity Management District had Gold and White marked patrol cars- thier sole duty was to give out pollution cites to cars producing smoke at a density greater that #10 on the ringelman chart. These pollution cops wore blazers on patrol too. Anyway, the common thread was the white doors on patrol cars used for primarily traffic enforcement.
I was told by a then long retired copper that this was done in beginning days of the migration of dustbowlers to California for agri-picking jobs. There many instances of where folks were stopped and 'fined' or 'bailed out' on the spot in the field by unknown 'cops' in plain wrapper cars for nebulous/made-up infractions of law.
I digress.
White: a contrasting color, yes perhaps, but I assert it was because of this obscure section in the now defunct CAC and became tradition because of it although most did not know how or why.
If anyone happens to know of a searchable verison of the defunct California Administrative Code that is FREE please post a link.....