I think other than this chart, pretty much all NOAA raster charts are already available as BSB
files. No need to reassemble when you can just download them ready-made
I am guessing this chart is not available as BSB
because it is so severely out of date.
When encoded as BSB, chart images use indexed colors. I wouldn't change colors much, rather I would run it through some filtering and then into indexed color mode to get a small palette (less than 32 colors would be good).
Of course a set of modern updated UKHO vector charts for BVI costs something like $30 (probably raster too)