He did not say buff was not real. He wrote that the color was Spar, (it) never was (called) Buff. Buff is a color...sort of the way purple is a color... Violet "is" purple. Lavender "is" purple. Magenta "is" purple. Purple is a broad term for a range of more specific shades. Buff is a broad term for various more specific shades of tan/cream/brown, and the term originates as Leo wrote. Ordinary folk who looked at ships of the era used a color term they knew. Buff colored coats were common enough to make the color term come to hand and be widely understood.

But, in the US Navy manuals of the time, the color called for was listed as Spar. If you told your Chief Petty Officer you were painting the ship Buff, he would see to it you that you painted that color until you dropped while repeating at regular intervals that you were painting it Spar.

As Ron notes, Spar itself varied, but it was the proper nautical term, and you would order it, or the ingrediants to make it, by that name, not Buff.

Per Wikipedia, the White Star Line called for the painting of their funnels to be Buff with black tops. That seems to be how it entered the nautical world:

https://en.m.wikipedia.org/wiki/Buff_(colour)

The Royal Navy used yellow:

https://www.sovereignhobbies.co.uk/pages/full-view-of-all-schemes?srsltid=AfmBOorSUQ-F-F1256QLTzmUNHROJLXublG_fVit3lAT4t5XDGmZgvPR

The modern US Coast Guard Spar color is:

https://www.scalemates.com/colors/colourcoats-since-2014--749/m22-us-coast-guard-spar-fs30371-enamel-matt--11805

With the usual disclaimer about computer monitors distorting color.

I don't know how closely to tradition the modern Coast Guard color is, but here is the Vallejo color Ron says is a match for Spar freshly mixed from the manual of the period:

https://www.scalemates.com/colors/vallejo-model-color--827/70877-gold-brown-acrylic-matt--36566

And here is the color Vallejo calls Buff:

https://www.scalemates.com/colors/vallejo-model-color--827/70976-buff-acrylic-matt--36651

With the same disclaimer about computer monitors.

Here is a "creamy" Buff color to British Army gear:

https://www.ima-usa.com/products/original-british-victorian-p-1888-buff-leather-slade-wallace-equipment-set-with-named-royal-marines-sargent-tunic?variant=26170197957

And here are more samples of Buff...

https://www.scalemates.com/colors/colourcoats-discontinued-2014--656/c07-buff-enamel-matt--789

https://www.scalemates.com/colors/colourcoats-since-2014--749/c07-buff-enamel-satin--11613

https://www.scalemates.com/colors/colourcoats-since-2014--749/narn46-victorian-era-buff-enamel-matt--30285

https://www.scalemates.com/colors/colourcoats-discontinued-2014--656/rn26-royal-navy-buff-based-on-snyder-and-short-now-superseded-enamel-satin--11577