Paris->Cologne

The best place to buy a train ticket is probably through the Deutsche Bahn site—https://int.bahn.de/en. It's easy to use. You can buy it online on your browser, or use the DB Navigator app. That is the official German train site, so you can usually get the the best price there. Plus if you purchase your ticket there, and download the app, they will send you push notifications of any potential delays or track changes.

In terms of timing, it's best to purchase it as far ahead as you can so you can get the best price. You also need to decide whether you want it to be completely refundable (the most expensive) or whether you should pay a cheaper price for a non-refundable ticket.

Normally the best way to travel from Cologne to Paris is with Eurostar, as they run direct trains. Only currently they don't run as many as normal because of issues on the line...
DB does run trains but you may have to change a few times en route...

I found a ticket on an app that looks pretty reasonable

What ticket on what app did you find? What is your actual travel date?

Yes, my question is also which app. There are some online travel agencies that present themselves as being official train company apps but instead are rip-offs. Use only the official ones given above for the safety of your money and credit card information although Trainline is a 3rd party and is reliable.

The app is called Trainline, travel is December 18.

It's pennywise and pound foolish to buy single train tickets from a third party. You think you are saving money, but you are creating distance between you and the "product." Same reason you should not buy plane tickets from Silicon Valley Duuuuudes. You want to be dealing directly with the seller (and product provider) when, say, the train fails to appear, or you get sick. You are so close to travel that you can't get a discount, so it's a choice between an online purchase that appears in a DB phone app, or going to another DB station and buying a paper ticket. (At home, you can print your discount ticket yourself, like we did Brussels-Cologne, which was really cheap 2.5 months in advance.)

With the slowest and cheapest last-minute tickets (a LOT slower), you aren't locked into a single departure, but there are also faster trains that the ticket is not good on.

https://www.ricksteves.com/travel-tips/transportation/trains

Thanks all for the super helpful information. I bought my ticket just now so all is good. I should mention that the Eurostar ticket was about 25% more expensive than Deutschebahn 1st class so I went with the latter. The schedule suited me better as well.

For future reference Trainline IS reliable and sometimes is easier to manage with a US credit card.

I’m glad you got something that works for you!

Good to know, and thank you again.

Because the fastest route is via Brussels, Deutsche Bahn is not going to be the best route planner. Trainline is useful, I have used it. Eurostar is probably best. Senior discounts start at 65.

Thanks all. I can't agree that using a third-party website is always pennywise but pound foolish. Sometimes they get discounts because they are bigger sellers of tickets. Not only that but the Trainline app seems to aggregate all the choices into one screen which is convenient and not that more much more. For example, the same ticket on Trainline is about six dollars more (about five percent) than Bahn.de and that could even be in part due to conversion rates because one is in euros and the other is in dollars.

I agree, using Trainline is indeed not always penny wise and pound foolish. I don’t pay a single penny more to use it and find the interface about a million times more pleasant than SNCF. To each their own.

Have a nice trip !