I've been digging around on the web - and got the following various extracts;
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From "Ships Nostalgia";

neil marsden
20 posts · Joined 2005
#13 · Oct 8, 2005
Unless my memory has failed me altogether the main decks on Clan boats were painted red, I recall somebody suggesting the reason for this was to mask the marks left by certain crew spitting their chewed 'beetle nut' about. If it did as much damage to steel decks as to their teeth perhaps it was a wise precaution!!

Neil
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From Google AI;

The decks of Clan Line ships were typically painted in colors that changed over time, primarily shifting from black to a "mahogany" brown or red oxide in later years.
Historical Deck Colors
The deck colors of Clan Line vessels evolved with changes in technology and paint availability:
• Early to Mid-20th Century (Steam Ships): Steel decks, particularly on ships with steam winches, were generally painted black.
• Late 1940s - 1950s (Electric Winches Introduced): With the introduction of electric winches, the deck colors transitioned to red, likely a red oxide anti-corrosion paint.
• 1960s (Later Motor Ships): Later motor ships, such as the Mahout and Markhor (1963), featured green decks.
• General Post-WWII Consensus: Many sources from personal accounts suggest the main decks were commonly painted a "mahogany" color, described as slightly darker and more brown than red lead, or simply red oxide. This color helped to hide dirt, rust, and oil stains common on working cargo ships.
Model Paint Suggestions
For hobbyists looking to replicate these colors, here are some recommended model paints based on general advice for similar historical ships:
• Mahogany/Brown/Red Oxide
o Tamiya TS-69 Linoleum Deck Brown Spray Paint
is noted as an attractive option that produces realistic results for deck surfaces.
o Humbrol 103
or
148
are also suggested as suitable colors, with an optional thin black ink wash to highlight details.
• Green (for later ships)
o BS4800 color 14C39
(a mid Brunswick green equivalent) was a Royal Navy deck color used into the 1970s and is a suitable reference for later Clan Line vessels with green decks.
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In Greenwich Royal Museum;

Can't pick up Link to image of model, but
search for Clan Ross in Museum's search function.
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