Colmar to Paris is probably a TGV operated by SNCF (there are no 1st class trains btw…) , so there would be nothing that SBB can do here anyway. And yes, in France the problem is that there are no reservation displays. So you do not know if someone indeed will turn up.
But most people do not care where they sit. So if you choose to sit together, and someone turns up claiming on of your seats just tell them what your original seat is, and suggest a swap. Most people do not mind.
Note that when you purchase tickets on SNCF you can indicate that you want a seat near someone who already bought a ticket, and the system then tries to book you together.