Car or train from France to Germany

Or is it possible to take the France car and return to Frankfurt without costing a fortune?

The only way to find out is to get a quote. My question is, do you need a car in Germany? Where are you going that you would need a car?

We would like a car to visit small towns for 3 or 4 days.

It's usually far more expensive to drop a car in a different country than you rented it in. But as Sam points out, the only way to know for sure is to request a quote or two (i.e. go online and do some searches).

https://search.ricksteves.com/?utf8=%E2%9C%93&query=Rental+car+France+to+Germany&button=

Perhaps there's some helpful info included in this Search.
Good luck!

Even renting from one city and turning it in at a different city in the same country can cost a fortune, which kept us from doing that in Norway this fall. Years ago, we rented in Portugal and returned it in Spain. That cost an arm and both legs, but it was much easier than returning to Lisbon, moving on to our destinations in Spain by another means, then again renting one or more cars. There were three of us splitting the cost, so that made the decision somewhat easier.

We’ve also been able to rent a car and return it at a different location within the same country for a relatively small, or no extra charge. That could depend on the rental company, and the city/cities involved. How far are German towns you want to visit from Colmar? Could you drive into Germany, come back to Colmar (or perhaps Strasbourg?) to return the car, then catch a train to Frankfurt to complete your itinerary?

I’d include Europcar in the search for potential rental companies, and possibly getting a break on costs.

Oops, sorry, Sam!

Can't blame spell check, but I will blame having posted at shy of 5 o'clock in the morning, and jet lag.

We would like a car to visit small towns for 3 or 4 days.

With good planning, you don't need a car to get to small towns. I've spent almost 6 months traveling in Germany (mostly) and Austria, 162 nights. Over half (57%) of my nights were in towns under 10,000 population, 85% under 20,000, and I've never rented a car. Germany has the most extensive rail system in western Europe (over 5000 stations) augmented by a very good bus system.

With the ease of buying and using the Deutschland Ticket, and the excellent rail service in Germany, I would think that trains would also be cheaper than renting a car, paying for gas, finding and paying for parking, and dealing with out of country drop off fees.