Frankfurt Airport

Which terminal do you arrive in? Google your flight and # if you aren't sure. Are you going with a regional train or long distance train?
I am at the airport all the time, but I know my way around. You don't, so everything may take longer, plus you have to figure in possible flight delays. If you are taking a regional train, it doesn't matter as the price is the same today as it will be next month (except there will probably be price increases end of Dec. or 1st of Jan.)

Where are you going? The more information you give us, the better and more helpful answer you will get.

There's no good answer. Planes are late, workers get sick, strikes happen, etc. I would simply plan to leave FRA whenever time permits. Where are you going that day? Will you have a Eurail pass or German Rail Pass? With this information, we can provide some train tips, perhaps.

I have updated my post to add more details. Thanks for the suggestion to do so.

I agree with Russ that there is no good answer. Booking ahead could save you close to half the fare, but the same-day fare might be less than you think. One way of handling this is counting back from the latest you're comfortable arriving in Colmar. A quick check of Bahn.de suggests it takes at least 3 hours and 2+ changes to get there. The airport has places to shower, as well as loads of restaurants where you can pass the time waiting for your train.
I feel like there is a way to buy your onward train ticket along with your airfare (maybe only on German airlines like Lufthansa and Condor?). Not sure why I think this or whether a flight delay would allow you to take a later train without penalty. This is worth looking into.

If you are using Rail&Fly (part of your airline ticket) or a "full fare" ICE ticket, you don't need to plan a "time delay buffer". You can use any train that day.

If you are trying to use a Savings Fare ticket, look out. Those tickets are train specific (day and time), and if your flight is too late, you could lose the ticket and have to buy a new one at full fare. On three occasions I've been an hour late arriving in Germany on a flight from the U.S. Once, after ticketing a flight to Munich on USAir, I started checking it's arrival times. It was 9 and 10 hours late on two days that week! Fortunately, it was on time the day I flew.

I never recommend people using a Savings Fare ticket the day of arrival in Germany due to the possibility of a delay. I might plan on spending a night or two in the local area before using a train-specific ticket.

In 2017, we arrived at FRA with an eventual destination of Bavaria, but we spent the first two nights in St. Goar, where we could easily get to on regional tickets. I wanted my partner to see the Rhein Gorge anyway. Then, 2 days after our arrival, when we knew we could make the specific train, we took an IC to Bavaria.

Or, if you can arrange your trip so the first night is not too far from FRA, plan to leave immediately on a regional train. Leaving immediately on a regional train might get you to your destination at soon as waiting hours to take a long distance one.

Using the ICE trains from FRA airport to either Basel or Strasbourg will minimize your travel time as well as the number of trains (3-4 different trains. But you are still looking at a train trip of 3.5 - 4.5 hours that same day. Uggh.

I'd be looking at the 13:51 departure from FRA with a change in Karlsruhe (15 minute layover) and then Strasbourg (17 min. layover) with final arrival in Colmar at 17:51. But...

1) The chances of missing the connecting train in Karlsruhe are good.
2) The chance that your flight may be delayed is unknown. A saver-fare ticket from DB will be subject to forfeiture if you miss that 13:51 departure.

You can look into a rail-and-fly ticket which provides a discounted flexible departure from FRA to Colmar that same day (contact your airline.) But that won't help you make the Karlsruhe connection.

My advice: Stay the night in or near Frankfurt. Catch your train to Colmar in the morning using the cheap saver fare bought for the date that follows your flight. This reasonable overnight time cushion accommodates a delayed flight and better ensures you can use the cheaper ticket - and you'll feel much more refreshed once you get to Colmar.

Which terminal do you arrive in? Google your flight and # if you aren't sure.

Just as help for travelers: every airport with more than one terminal has a list on its website which terminal an airline is assigned to. Example FRA: https://www.frankfurt-airport.com/en/flights-and-transfer/airlines-a-to-z.html

I would not take the ICE to the main station in Frankfurt. Take the S-8 or S-9 or any regional train that stops at track 1 in the regional station. They all stop at the main station in Frankfurt. Cheaper than the ICE. The regional station is directly under terminal 1, and faster to get to then walking all the way to the long distance station where the ICE are.

If you are taking a regional train to Colmar, just get your ticket after you arrive. There is no price difference getting it ahead of time.

You stated you have to take "An ICE train to a main terminal."

Do you mean Frankfurt or another station?

If Frankfurt, I'd go with Ms. Jo's advice to take the S bahn over any other train as the station is closer to the terminal. If you are going to another station directly from Frankfurt Airport then stick with your schedule.