Hi Nick,
Re your watercolour and your HMS Kent.
If it was my model, I'd go for it, and use the watercolour as a basis. The artist was there, at the time, and even if he did paint it later, from notes etc., his memory for colour would be as close as damn it.
I am building a couple of NZ river steamers from 1864, and have watercolours painted by artists who traveled on the boats, and painted the scenes while on the river, at the time.
Details, and colours depicted, are better than the only existing descriptions from the time, as text is open to interpretation. I have one photo from the same time period, and matches the watercolour but in black and white. Historic artists where the cameras of the day, and even if they simplified, or left detail out, what they did draw, would have been what they saw.
Go for it and enjoy the build
cheers
vnkiwi
ps even writers and researchers get things wrong, or miss things, and even the press at the time often get it wrong, getting graphic illustrators in a different country (in my case) to do an illustration from a written description sent by mail. You'd be amazed at the various different pictures possible and they all wrong, but accepted by historians as accurate.