From the corners of my memory, I recall that Paul's standard greeting in his letters is an transformation of typical Greco-Roman letter greeting, greetings. Paul isn't simply saying, hey there Colossians, how's it goin'?
A blog post I came across discusses Paul's actual greeting with an interesting twist. The phrase is usually translated as "grace and peace to you" but in the Greek it is literally "grace to you and peace." Theologian Gordon Fee thinks this is significant.
Grace is unmerited favour. Paul elsewhere says that salvation is by grace as a gift, and not earned (Eph 2:8-10). Paul constantly reminds his readers (and us) and being saved (in all of its significance) is about free gift. It is as Miroslav Volf says, Free of Charge. Grace is giving freely, being gracious and not just doing gracious things. Christ's grace reminds us of our own need to be gracious, as well as grateful for the grace extended to us.
Bonhoeffer reminds us that grace is not cheap, even though it is free. Grace brings with it the call to sin no more, and the means by which this might happen. Amazing Grace might be a very oversung hymn, but it is an inexhaustible concept.
For Fee, Paul is saying that peace comes after grace, it is a consequence of it. Elsewhere, Paul says that peace is a result of being justified by faith (Rm 5:1). The implication is that without this, there is no peace. Romans 5:10 says that those apart from this faith are enemies. Grace makes enemies into friends of God (Jn 15:9-17). This is why we are to love our enemies and pray for our persecutors (Mt 5:43-48).
The Greek word picks up on the Hebrew idea of Shalom. Peace is something that encompasses all of life. For example, in Leviticus 26:6, peace means freedom from enemies and wild beasts - an agricultural paradise in an age well before people were more willing or able to live along side wild animals. That this peace comes from God reminds us that the material and spiritual are not meant to be separated - ultimate peace is all encompassing.
Finally, peace with God is not merely rational, not fully understandable and transcends our understanding. A sense of peace is not merely a "warm fuzzy", but that which keeps our intellect and affections in Christ.
May your day be filled with grace, and peace.