I have searched Family Search, FindMyPast, Ancestry and others but the only marriage that comes up for Duncan Buchanan is;
Name: Duncan Buchanan
Gender: Male
Marriage Date: 22 Jul 1721
Marriage Place: Kilmadock, Perth, Scotland
Spouse: Grissel McGruther
Oh dear, oh dear.
First, you aren't looking in the right places. FS, FindMyPast and Ancestry (and MyHeritage) have at best an
index to the original records, and at worst a tissue of invention. See
https://www.rootschat.com/forum/index.php?topic=714261.0 Second, just because there is only one surviving record of the marriage of a Duncan Buchanan does
not mean that he is the right Duncan Buchanan. There are many other Duncan Buchanans* whose baptism records have not survived, and the further back you go, the more gaps there are in the records.
(*as an example, there are 15 surviving baptism records of Duncan Buchanans between 1650 and 1710; and 72 surviving records of baptisms of Buchanans, whose father's given name was Duncan, between 1700 and 1750. None of these surviving records names the mother as Grissel or Grizel, barring some really weird spelling variation that I haven't thought of.)
If the original record, available at
www.scotlandspeople.gov.uk, contains additional information that positively identifies Grissel McGruther's husband with the son of James B and Elizabeth Stewart, well and good. Otherwise he's almost certainly not the man you are looking for.
Does the Red Book give a source for the statement that Duncan Buchanan married Grissel Robertson? If not, your best bet would be to ask the compiler of the Red Book what his source for this information is.
Failing that, there could be something in the National Records of Scotland, for example sasines or a marriage contract. Unfortunately finding it without some hint where to start is a bit like looking for a needle in a haystack, as not all the surviving records from the first half of the 18th century have been fully indexed. See the catalogues of what has been indexed at
https://www.nrscotland.gov.uk/research/catalogues-and-indexesAnd good luck, because you are going to need it.