I have attached a Chevrolet reference documenting that three 1931 models were available from the factory with colored fenders. My understanding is that the other chassis sheet metal was also painted a body matching color. The paint was sprayed only on the show surface and the remainder of the parts were standard black. The color was the moulding color not the major body color.
My interpretation of "regularly equipped" together with "at an additional $15.00 List" indicates a factory available option. If it were all then it would have reflected in the regular price.
Here is one I believe was done in this fashion. Look very closely at this photo. The Wheels, Fenders (don't look black to me) and back portion of the body are all the same color. In this case I would fathom Cellini Green.
My 32 Special Sedan has fenders and splash aprons that are painted the upper color of the body. Previous owner told me the car came from the factory that way. He said that was an option in 32 but because of the additional cost few cars were painted that way.
My special sedan is painted code 81 which is one of the codes reserved for those three open cars. Fawn brown ( including fender tops, coffee cream, with swamp Holley orange wheels. I know my Chevy is incorrect as far as OEM goes but I get constant comments about how beautiful the colors are. It also doesn’t hurt that the paint is basically flawless. My buddy Moses did an incredible job painting it.
Chistech, Yes your Special Sedan is a bit more "special". If you like it keep it. Those chocolate brown fenders set off the body even more than black ones. My six wheel '31 Phaeton has same combination with black fenders. Looks good but not as good as will my DeLuxe Sport Roadster when I finish the paint job.
As far as I have been able to determine 1931 was the first year to offer matching colored fenders. There are a few DeLuxe models in 1932 that also had that option.
I agree with your color choice (code 81) as well. I have my 31 sports coupe in the same colors and always get some great compliments on the colors and how bright they are in the lineup! Mine has been done since the late 70s and still very presentable.