Columbian is the name of a pattern mutation that restricts the black mainly to the hackles, tail and on the primaries.
Visible examples of columbians are Light and Buff Brahmas(in silver and gold versions, respectively), Columbian Rocks etc.
It is also present in a great many breeds or color patterns, but not so visibly, it is also in Buff Orpingtons, RIR, etc but you couldn't easily tell by just looking at them. It is used in these because it is an excellent aiding gene in keeping the breast and most of the body "clear"(free of miscellaneous lacing or markings).
A partridge roo or BBR roo with a black breast does not have columbian. If columbian was added to either, they would be "turned" into a mostly solid buff/red bodied bird with black on hackles and tail.
I don't remember all the different genes that make up for mille fleur though.. seems you are saying columbian is involved(would make sense for the clear body). So if this is correct, your mille fleur already has columbian in him. As I don't know the whole information about mille fleur pattern, can't answer your question much otherwise, sorry.