When I check my Master Parts Catalog for the sill plate (13208) Ford splits between the 66-67 and the 68-69. Nothing about color stated in the text description. However, when I check my AutoKrafters catalog, they sell the same plate for 66-69 and sell the stickers separately (or you get them free with the plate) They show black stickers for 66-67 and blue for 68-69 I have a 69 and I’m pretty sure mine is blue. If the plates are physically interchangable from 66 through 69, my guess is that the only difference in the Ford part numbers is the sticker, and from what I can see, black would be the correct sticker for you.
I looked again and the plates I have seem to be original. From what I can see, the stickers are correct, maybe the option 2 above was a Fairlaine/Torino thing because I can’t find a reference for them in reproduction parts for Mustangs, Cougars, Broncos, and Full-sized Fords in my searches today. In all cases the black logo with “Product of” and “Motor Company” were correct for 1966 so that is what I’d get if it were mine.