Hello, Keith;
I think what the author of the article was doing when he referred to "yaller kiver" was using a country dialect for "yellow cover". That, I think, was referring to the popular dime novels of the day which usually had yellow covers. In other words, what he was saying was the deeds of the Jayhawkers would be recounted and dramatized in those early pulp publications.
Anyway, that's my take on "yaller kiver".