Is Navigo decouverte covered in Strasbourgh and colmar town? If not, what's the cheapest transportation?
Strasbourg & Colmar are outside of the Paris region (Ile-de-France) and are not covered by Navigo Decouverte. You can see what is covered by Navigo in this Paris transportation zone map showing zones 1-5 in the Ile-de-France region around Paris.
There are TGV, Ouigo, ICE and Ter trains that travel from Paris to Strasbourg / Colmar with prices during high season (Christmas holidays) as high as 120€ but also as low as 19€ if you book ahead by 4+ weeks.
You can search for trains from Paris to Strasbourg / Colmar on Oui.sncf which is run by the official French train group (SNCF).