Train travel

Florence-Venice

Easy https://www.italiarail.com/pages/routes/florence-to-venice

Venice-Colmar

Quite easy - 2 changes (Milano & Basel)

Venice-Milano by Frecciarossa or Italo

Milano-Basel by Trenitalia

Basel-Colmar by SNCF

One important thing to be aware of:

if you are planning travel for a few months in the future, train schedules may not be posted yet. I was looking at Berlin to Prague trains for next summer and didn't see many; then I had the idea to search for trains in February and there are tons of trains. Practically hourly or more. It turns out that the trains for next summer are not yet available.

The other thing is that to get the cheapest price, you might want to buy them in advance and not wait until the last minute. This can be a bit of a contradiction since tickets are not available too far in advance. So I suggest you figure out when they go onsale - is it 2 months? 3 months? and then start looking right at that time.

Train travel in Italy - probably in most of Europe - is fantastic. I highly recommend it.

Have a great time!

ps: I love Venice and Florence (though both are incredibly crowded) but thought Colmar was just a "meh". But that's me, perhaps you will love Colmar.

And be aware that Venice to Colmar is a 9 hour journey by train.