Agreed, Masato, I don't know of any 19C references. The references to Tubman's use of the song is not backed up by any literature (that is why I emphasized the "it is said").
The version "God's A-gwineter Trouble The Water" could be an assembled song, part old spiritual and part (the chorus) a relatively recent addition, but only the subject matter, not the form, seems to appear in older spirituals. Unfortunately, lack of documentation is true of a number of "old" spirituals and songs.
Jeannie, color was used for separation of groups or characters and for emphasis in a number of the songs. This was not considered "corny" by those who sang them.