Did I get scammed in Colmar?

No you weren't scammed. Sitting at a table instead of carrying out or standing at a counter has a higher charge. But it's not a city tax AFAIK, but a higher VAT.

Italy also charges more for a ‘sit down coffee’ at a cafe versus ordering your coffee etc at the bar and standing there to have it.

This is a common disparity when you eat in rather than carrying out something like baked goods. I've run into it in multiple countries. It might be due to a difference in the tax rate; I had assumed it was an extra charge by the bakery for taking up one of its seats and using its tableware (with attendant dishwashing cost).

NO scam !! It is fairly common through out Europe. Sit down is more expensive that take away. It is not a coperto charge. Basically view it as a charge to use the table.

If the cafe scammed you for just €1, they really need to up their game.

In addition to Europe, I’ve seen this a few places in the U.S. Mostly independently owned coffee shops that have an eat/drink-in vs. takeout price.

Don’t visit California if you don’t like this type of “scam.” It’s a bit nuanced when the tax applies, but when you are asked “is it for here or to go” it isn’t because they care about how you plan to enjoy your food! Looking forward to visiting Colmar next year!

Same here. There is a different sales tax for eat-in versus carry-out.

Cafés typically have 3 prices levels: consumption at the bar, an indoor table, or on the patio. The price differences are usually displayed somewhere but are inclusive on any bill, they are not added on separately. Taxes are always inclusive within whatever price you see.

Hotels will add the city tax separately. It is usually a couple euros per night, but I have never seen, nor heard of any individual tax charge added to a food bill. I have been to Colmar 3 or 4 times and I have never seen this. Café prices must be posted, I am suspicious that this is an effort by a misguided café owner to increase revenue. If he wants to charge more for on premises consumption, the price needs on the menu, not an add on surprise.

Yes it's a scam.
As Tocard says it should be on the bill and also be clearly displayed on site. BTW, there is no difference in TVA in this matter https://bofip.impots.gouv.fr/bofip/7204-PGP.html/identifiant%3DBOI-ANNX-000495-20240207

This could be a matter of 'droits de voirie' if you were sitting outside and the owner had to pay a permit to use a temporary terrace during this festive period. But even in this case it should be clearly posted.

Perhaps check your outrage. Cheating customers carries some non-trivial risks.

If you think this business is running an ongoing scam by sneaking an extra €1 into customer's bills, would it really make sense to run those risks for one Euro? Really?

I would suggest you might consider that perhaps the person working the cash register - almost certainly a non-native English speaker - that was quoted by the OP was simply busy and just grasped for the right word in English without stopping to think long enough to give a perfect legal translation of the word "fee" or "surcharge" (or some French term most of us have probably never heard) and just used the widely-understood and frequently-used word "tax" because it's meaning was close enough: a required, small extra cost that nobody likes to pay. Perhaps the minor distinction between the terms was not the cashier's primary focus at the time?

Hanlon's Razor: Never attribute to malice that which is adequately explained by stupidity.
The Bill Gates Corallary: "I feel pretty stupid that I don't know any foreign languages. I wish I knew French or Arabic or Chinese."

How's your French? Could we all spit out the proper French word for "surcharge" at a moment's notice? I couldn't. I'd probably just call it a "tax" too and get on with the next customer.

Try running this through Google: "French word for tax." For me, it returns this:
l' impôt: tax, levy, impost, imposition, excise, scot

(scratching my head: "scot?!"...)

or some French term most of us have probably never heard

Supplément or majoration ;-)