They need black so.. buff columbian would further hinder any color progress. I think mottled is your best bet for that trio. While you're outcrossing, you don't want to waste any time (like years) on mediocre mottleds at a swap. I would hunt down the best from either Jamie Matts or someone that wins with them at shows pretty regularly. The difference in type between what some call show quality stock and actual prize winning cochins can be quite drastic.
But, you need to think ahead. Once you have that next generation, you need to have options. Putting them back will destroy any type you've gained. Putting them together will give you more of the same.. maybe some color improvement. ...so.. I would also get the bc's and cross them with the mottleds at the same time. Now you have things moving forward in both type and color and will have the option of crossing the better typed birds with excellent typed birds that now have slightly better pattern and won't destroy what pattern you've created..
They did turn out very pretty. I can tell the slight difference in type by the tooshies on the darker pullet and the cockerel from the above photo. Maybe Mildred's tail is squished but, she looks like she has a point back there.
eta: If you ever shop cochins online, always ask for the aerial photo. That angle can show the width of the bird that can be hard to judge by the side profile.