The New Testament has no record of Paul visiting Colossae and his letter to the believers there suggests that, at least at the time of its writing, he had not done so: "We always thank God, the Father of our Lord Jesus Christ, when we pray for you, because we have heard of your faith in Christ Jesus " (Colossians 1:3-4, italics added). That Paul had only heard of their faith suggests he had not visited. Colossians 2:1 refers to "...all who have not met me personally", which only strengthens the case against Paul's actual presence in Colossae.
However, analysis by Dr Mark Wilson confirms the very real probability that Paul passed through the Lycus Valley and would not have been unfamiliar with Colossae, even if some of the Christians there had never personally met him. See here.
Whatever the case, visiting the site is justified by Paul's knowledge of, and response to, the issues facing the young church there. While the site itself is not spectacular its proximity to the more extensive and spectacular sites of Laodicea and Hierapolis, both also mentioned in the letter, mean that a visit to the area is well worthwhile.
All that I have seen of Colossae is the relatively small unexcavated mound in the midst of a fertile valley: it is an exercise in "being there". I have visited this site with each of the study tours I have been involved with.
Click here for a Google Earth view of the 'mound' of Colossae.
The unexcavated mound of Colossae. A rough path from the location sign leads to the top of the mound.