if we can prove we have the capability to create a successful colony on mars, doesn't that prove we should be successful at doing it anywhere?

We don't need to be on Mars to show that we can live there. We can show that from Earth because we know a good deal about Mars, however every planet (local or not) is a little different so proving that we can live on Mars doesn't prove that we can then live anywhere because they'll all require different problems to be solved. Being able to get out of our solar system is more important I think than colonizing Mars for the sake of learning about Mars because I think we know already that we could do it. Propulsion systems mean more to me than living on Mars I guess.