http://www.dmv.ca.gov/portal/dmv/det...3/c1/a9/5201.1
https://www.dmv.ca.gov/portal/dmv/?1.../d3/c1/a4/4464
(c) A person shall not erase the reflective coating of, paint over the reflective coating of, or alter a license plate to avoid visual or electronic capture of the license plate or its characters by state or local law enforcement.
4464. A person shall not display upon a vehicle a license plate that is altered from its original markings.
So potentially it could be two strikes. One for obscuring readability by painting over the reflective surface, and another just for having altered the plate.
I'm not saying I agree with it or not, but it does seem that painting your own plate would be breaking the law.