Hi Steve. I think what you say has merit. There is certainly some "wiggle room" here.

I'm just a novice in the subject myself but it appears that iron boats tended to be coal tarred, because that was a more durable & effective rust-preventative finish than paint.

As noted above, the color of coal tar could vary widely, so we may need to wait for news from the lab before we can be certain what color the Hunley was at the time of her sinking. Meanwhile i wil see if i can find a date for the Cook photo.

From what i can see (mainly discussion in Hunley.com Newsletter #47) nothing is known about the date of the Cook photo.

I'm not sure if an original plate or tintype even exists - the known images seem to all be fairly degraded reproductions. Something for further research. Cook was a well-known Charleston photographer & something could be buried in his archives.