No problem to do at all. 

If you’d like to do just these trips I’d opt for the cheapest rail pass. Should be an Eurail Global pass for 4 days in a month. 

Even if it's possible to do the cross border journeys with an additional ticket when holding a Swiss pass this option is much more expensive.

Most of the time a Swiss Pass is more expensive than a Global Pass anyway, but I guess it depends on the number of travel days and your country of purchase.

You can actually keep most of your travel within Switzerland if you want, by travelling over the Bernina Pass to Tirano (highly recommended journey anyway!). Then you only have a short section to Varenna and back where you need to buy a ticket.

Similarly, for the trip to Colmar, it’s only a short section that wouldn’t be covered by a Swiss Pass: Basel-Colmar and back.