In a previous essay on the authorship of Ephesians (link 1 below) we proposed the thesis that that letter was composed by Paul's son in faith, Timothy, who had been influenced by his fellow Ephesian transplant, the apostle John. In the case of Colossians, we would like to tender the same proposal -- and this time it is easier, since Timothy is clearly one of the people involved in writing the letter (1:1).
In light of this we may now consider a few arguments against Pauline authorship of Colossians and see if they have any parallel to the arguments against Paul writing Ephesians.
One factor that can certainly be paralleled is that Colossians contains hymns and liturgical material. As with Ephesians, unusual diction can be attributable to the use of hymnal and liturgical material from within the church. The Colossian hymn (1:15-20) is an obvious example, as well as 2:13-15 [Martin, 32].
External evidence. Here again attributions by patristic writers, by orthodox and heretical alike, are unanimous in favor of Paul. (For the importance of this, see Glenn Miller's item, link 2 below.) Irenaeus, Tertullian, and Clement of Alexandria all agree it is Paul's, and there was no dispute over its authorship. However, there are very few indications of allusions to Colossians in early second century works; Justin may allude to it [O'Brien, xli]. Marcion did include it in his canon.
Textual evidence. Again, if this and the patristics were all we had, the case would be open and shut. All manuscripts agree that Paul is the author.
We now move to the evidence of vocabulary and tone, and we begin by repeating some points stressed in our study of the Pastorals and used again regarding Ephesians. As we noted there, word choice and writing style are NOT suitable criteria for saying that a person did or did not write a particular piece of literature - especially when we are dealing with writing samples as small as Colossians.
In this regard, conservative scholars rightly cite the work of Yule [Knig.PE, 39; Oden.12TT, 13], who notes that samples of at least 10,000 words are needed to make such determinations - and Ephesians is rather short of that mark.
What about a scribe? The odds that a scribe did most or all of Colossians is quite high, and because he is named, Timothy is an obvious choice, and because Paul is in prison (4:10) and would probably be chained and unable to write himself. We will not repeat here our earlier points regarding the effects of such a scribe; these are found in the linked essay above on Ephesians.
However, we may add the point made by Lohse  that if we posit a scribe, then why not posit someone writing even later? The reasons again are 1) the direct attributions; 2) the widespread use of a scribe in the NT times; 3) the church's care against pseudox (see Miller's link above).
Word choices. Colossians contains over 34 hapax legomena (words not found elsewhere in the NT), not out of line considering the amount of liturgical material. Interestingly and in line with the idea of Timothy as scribe for Ephesians and Colossians, 10 words are found only in Colossians and Ephesians in the NT, and 15 more are found in these two letters alone of letters attributed to Paul.
As noted, though, the totals here are hardly extraordinary; Galatians for example has 31 hapax legomena.
Vocabulary usage. There are a few unusual words cited by Lohse (who nevertheless admits that such indications are not decisive). These are outside the probable liturgical and hymnal material:
At the same time, no matter who the author is, that he confronted a legalistic doctrine makes it "odd" that "law" is never used anyway.
There are also so many Pauline words and phrases in Colossians that it has been posited that it is a letter composed partly of interpolated Pauline fragments [Martin, 38] put together by the author of Ephesians.
There are also the usual charges of Pauline phrases used in a non-Pauline sense:
Theological differences. On this account we are served the following arguments:
As with the Pastorals this is merely assumptive reasoning. There is no reason why apostles coming out of Jewish backgrounds would not readily (and within 20-25 years.) be able to embody church traditions, or as they aged think about successors. (Even at the tender age of 34 I had thoughts of who will take the reins of Tekton when I pass on. Is it so hard to ascribe such simple thoughts to the Apostles, especially in a day when the average lifespan was a mere 35?
But as it happens, this reads too much into the text anyway; 4:12 gives no sign that Epaphras is Paul's "successor."
In conclusion: The wrangling over Colossians is not as deep as it is over Ephesians, and thus it is easier to hypothesize that Timothy wrote this with Paul's direction.