CDG to Colmar

Doable as long as the flight arrives at 8:15. I'd wait to buy the ticket once you are sure that the flight is arriving on time.
Download the SNCF ap and either buy the ticket once the plane takes off on time or while you are in the passport control line.
Make sure your phone or tablet needs internet access to do this.

There is a direct train from CDG to Colmar at 11:22

I believe you must take a connection at Strasbourg on this route.

I'm going to give you a different opinion. Those trains sell out and I don't think you can wait until you're in the arrival line at CDG.

Buying a train ticket for 3 hours after your arrival is a risk - but I think it's a reasonable one. I've done it on arrival to CDG and it worked out. But - we had no checked bags to wait for and were some of the earlier people off our plane.

You do need to transfer in Strasbourg.

There is always a risk in booking onward travel the day of arrival. 3 hours should suffice assuming the plane is on time; planes from the US are usually a bit early BUT ours was two hours late last fall -- so not always. Immigration can be very slow. It took us two hours last spring and 45 minutes last fall.

I'd book this train and just live with a late plane. it is alas too late to get the cheaper tickets.