IANAL

A 5-second Google search says a right on red is legal in California, except if the red light has the shape of an arrow, or if the intersection has a sign prohibiting right turns at a red light.

Source: CA DMV Driver Handbook

This is going to be the key to the argument you presents in court. You need to take photos of the intersection, and make sure the red light you were stopped at did not have an arrow shape. You also need to make sure you were not in a dedicated straight lane, with a dedicated right turn lane to the right of the car's position. You also need to confirm if there was a sign at the side of the road specifically stating right turns at a red light are prohibited.

If you can give proof in the form of photos of this intersection showing all the boxes are checked for a legal right on red, this is an easy ticket to get out of.

/u/GreySwirl's advice is bullshit, because the police do not need to know the laws, they leave that up to the prosecutors. You should absolutely fight this, because this is a brain dead easy ticket to get out of, assuming you didn't violate any of the checkboxes I mentioned above, and followed the ones listed on the DMV Driver Handbook I linked.

In fact, you should use that DMV handbook as evidence as well, since that will give you a series of checkboxes to present to the court for why your turn was legal.