FEC used numerous paint schemes but the basic car was boxcar red I believe (are you aware of any experimental paint schemes in black or some other colour?). What paint scheme are you planning on - perhaps The Speedway to America's Playground scheme?
If you've not come across it yet, I have a book about the Early days of the Florida East Coast Railway titled "Last Train to Paradise". Although it mainly documents the Key West railroad and it's demise, it does have a number of photos (but they're all black and white). I'll look for it and if there is anything of what you're looking for I'll let you know.
I recall that it was a fascinating read on the whole history of the FEC and Flagler's push to Key West.