The term “golder” does seem to be rare. I also felt initially, that it was awkward to the ear. I did find the following, though:
Macmillan’s magazine, Volume 67 edited by David Masson, et al., page 62, 1893:
And then I speculated that maybe the sense was simply one of ‘color’, rather than as Dave hinted at above, “material composition”:
[...The OED doesn’t mention adjectives that take the ending -en,] which is added to nouns to form an adjective denoting material composition, e.g., golden, earthen, woolen....
There are many other color words with -er suffix:
Micrographia: or some physiological descriptions of minute bodies made by ... by Robert Hooke, page 66, 1667
“whiter”, “browner”, “blacker”, page 214:
Another work, Theatrum botanicum: the theater of plants : or, An herball of large extent ... by John Parkinson, 1640
Has many -er colors, in fact, more than Hooke’s work; for instance, it includes “redder”, “paler”, and “purpler” and probably more that I missed:
Of course we have “lighter” and “darker” also as color qualities.
My speculation is that if “gold” was considered to be *simply* a color, then “golder” might *seem* to be correct. Consider also, the audience for which the article is written--it appears to me to be written with school-age children’s accessibility and utility in mind.