I quote from the text of a e-mail response I received from a LaDOTD public information officer:
Dear Mr. (name withheld):
All of the original green and white state route marker signs will be replaced by the new black and white signs over time. Green ink ismuch more difficult to use than black ink, which results in higher fabrication costs. Although there is no written DOTD policy on the color of state route markers, the Federal Highway Administration's Manual of Uniform Traffic Control Devices encourages states to adopt a uniform state marker containing black numerals on a white area
surrounded by a black background.
Public Information Officer
Department of Transportation and Development
So it is definitely official. The black on white LA signs are the new 'official' markers and the state will phase out the green and white shields.
Please explain why we have been using green ink on the state highway shield since 1960 if it is more expensive to use. In any case, if this is the stated reason, then I guess I will grudgingly concede their case. Spending the highway money on actual highways, as opposed to highway signage, is a worthy enough goal. Also this places the state in line with the FHWA standard.