There is no off the shelf color I know of. You can get a fair color example by Googling "California license color". I can't imagine painting the letters with anything but 1Shot sign painters enamel which would require mixing yellow and orange probably. If the black you have used isn't enamel you probably will have a problem.
Personally the license plates I have done over the years was using the old Rod & Custom or Street Rodder method of painting the letter color first and then painting the background color on top and then sanding the letters back to the letter color. 600 of finer paper on a popsicle stick produces excellent results but it takes time. Cover that with clear. I don't know if this will work with water based paint. This worked really good with no longer available lacquer and I have used Krylon rattle can "kinda lacquer" and I'm sure their enamel or Rustoleum would work if covered with clear. Again it is necessary to find 2 colors to mix to get the right shade. It is possible to remove the rattle can paint by turning the can upside down and spraying all the propellant out (there was a utube video of two guys doing this while smoking!!) and then drilling holes to remove the paint and using that in a touchup gun. I ran across rattle cans 3 years ago when I did a black & yellow/orange plate for our teardrop trailer that must have had a pressurized bladder in them. The two Krylon colors I used are no longer available. There are companies like Automotive Touchup that will put lacquer car colors in a can.

 

Already have 1 Shot "Chrome Yellow" and "Imitation Gold". Both colors were advised from 67-72chevytrucks.com some member. Problem with them any "ratio mix" doesn't get the color thats needed.
Black Satin "Rust-Oleum Enamel" is the background color I'm using ( don't want it shiney ).
Using a hard roller wrapped with a T-Shirt material will work also I heard the"single wrap" of T- Shirt material is just right it wont soak up too much paint but just enough of it to lay down a perfect layer.
Was told that RUST-OLEUM's FARM & IMPLEMENT CATERPILLAR YELLOW is as close as you can get from * * * " Any Off The Shelf" * * * Store Bought spray can. RUST-OLEUM Sunburst Yellow" and "Sunrise Red" both gloss Oil-Based ( Cant get in Calif) brush on paint will mix ( I was told) to give the right shade of color for whats needed. I haven't tried those color combinations yet.

 

I came across someone saying that Krylon School Bus Yellow was good, but I'd do as suggested above and expect to mix colors w/ 1Shot, which seems to cover and flow better than anything for hand work.

Most restored plates are easy to spot because of the job done on the lettering. Tricky stuff, to do right. I believe that originally the letters were done with a roller and I have thought about experimenting with some-type sponge rubber rolling process or tool if I ever did one, to really replicate the way the outline flows around the stamped shape of the characters.

 

I also have heard School Bus Yellow will do it, problem is NO Home Depot type of home Center's carries it. The roller method with it wrapped with one layer of a T-Shirt material does great lays down a perfect layer of paint ( I was Told ).

 

I'll try and send a pic of a license plate finished per July 74 Street Rodder magazine

 

Not really for sure if that Yellow color is the same color used some 31 years later, anyway a Computers rendition of colors arn't quite correct.

 

The pic was just intended to show the results of the Street Rodder process. Definitely a different "yellow". From around the late 20's to about WW11 in California you got new license plates every year with the same numbers but alternating between black/yellow and yellow/black. The "yellow" always seemed a little bit different some more orangy. I think the first year for metal year tags was 1948 on a 1947 plate. I think 1964 on a stamped 1963 plate was the first year of stick on year tags. I have 3 1963 plates and the buffed paint color to me is pretty close to the on line pics. Part of the problem is do you want the sort of dirty yellow of a used plate or the satin new plate color. No matter have fun with your project.

 

The plates when finished wont be shinny at all as I will make sure the shine is dull. Probable will clear coat ( maybe) them but will use a Matt Finish or Semi Gloss Clear with plenty of coats so I'll be able to lightly sand the shine down. That's my story and I'm sticking to it, LOL

 

It might be hard to "sand" among the small letters & numbers. The American Restoration guy uses corn starch! No big deal if it doesn't look like you want you just spray on another coat of clear.

 

To sand with I'll use a Painters red/gray Scotch-Brite pad to dull the shine.

 

My two finished Claif. plates has these colors used, for the yellow/gold color it comes in at 99% for a Great match.

 

Nice job, really nice job considering what you started with!

 

Of the two together the one on top is the plate that was pretty beat up in the first place. It did come out pretty good at 10 feet away but that's all right with me.