flgarden -
About Columbian gene - since your roo is a sport, he likely carries just one copy of Co, it is dominant. It is not really visible on the Birchen allele. In theory the Birchen allele nulllifies the normal effect of pushing black to the extremities - wingtips, hackle and tail (like Delawares). Though I've read that the Dutch use Co in BCM to control overmelanization, genetic theory says it doesn't do anything. So our BCM can carry Co and we don't know if it makes any difference in the formation of the ideal SOP bird!
On the Wheaten allele you try to re-create the "Ermine", or the silver version of Blacktailed Buff. Basically like a Silver Sussex, all white with black in the hackle stripe, tail and wingtip.
You would need the silver from Birchens. That's the only unbarred silver based Marans. But it would be really hard to segregate the Co gene from a Birchen cross because it would be invisible on the dominant Birchen allele. It would take 3 generations and 16, 32, then 64 birds to get the right genetic combination for pure silver columbian! And that is if you happen to select the right birds that are carrying Co.