The register's date of 1850 is the date of a printed book, and a longer trail exists. A Scottish Tartans Authority note on Buchanan describes a cloak of about 1800 to 1810 in the collection of National Museums Scotland, in hard tartan made by William Wilson and Son of Bannockburn. In that piece the pattern is already asymmetric, the red, green and yellow blocks are close to equal in size, and the blue is Wilsons' light blue, not the mid to dark blue common now. The cloth also contains a weaving error in the light blue.
The same note says that small asymmetric checks of this kind were among Wilsons' fancy patterns, often identified by number, with some named later after a place, person or event. It says the pattern was known as Buchanan by 1831, because Logan's 1831 book includes it, and that Wilsons were calling a variation of it Buchanan by 1838. It calls the pattern unusual among clan tartans for being asymmetric and for its amount of yellow, which it says was uncommon before the Sobieski Stuart designs of the 1840s.
This page read the note through a search extract, not the full PDF, and has not seen the cloak, so these dates are level C pointers to be checked against the museum record. Two things follow if they hold. The light blue on the cloak matches what the register says the Smiths replaced with mid-blue in 1850, which supports the register's account of that change. And the name Buchanan was in use for the pattern by 1831, nineteen years before the register's date. None of the sources read says who gave the pattern the name or whether the family had adopted it before the Smiths printed it.
The hunting version has a separate documented source. Kinloch Anderson, a kiltmaker, says it comes from the Scott Adie collection, a book of manufacturers' samples dated 1850 and sold at auction about 1960, and that the samples are thought to represent the tartans on sale between 1840 and 1850. That is a commercial, level D statement. It places the hunting colours in the same trade decade as the Smith printing and does not show clan use.