If they're not a pair, take the blue that looks most Columbian and breed it to the true Columbian. You can slowly work the Columbian pattern into them. Thankfully, it is incomplete dominant, which means you can tell how many copies they carry. If they carry 2 copies, they will be all white except for the hackles and tail. If they have only 1 copy of the gene, the blue spreads through the white on their bodies. If they don't have any copies of the Columbian gene, they will only be blue. Unfortunately, if you have to breed a true blue to the Columbians, that also means you might end up with some pure blue chicks which do not have any Columbian patterning. If that is the case, just cull those out of the program. Again, as your numbers grow, you'll be able to remove more and more of the leaky ones or pure blue ones from your pen and work toward the goal of only Columbians.