In some states only emergency flashers and turn signals can be yellow.

As to the OP question I don't know about Califonia but I have never heard of anyone being pulled over for having a flashing headlight. This of course won't stop some enterprising cop from attempting to ticket you for this. I bike in a very urban area and don't actually have to use my headlight to see most of the time so I keep it on flashing 80% of the time when I use it.