I'd rate him third behind Ashford, who's only 20 and I believe has more potential due to the significant levels of football intelligence he shows, and Robertson, who is a year younger than Colwill and I reckon is potentially more valuable in the long run.
Although it's by no means out of the question, I'd be surprised if anyone would be willing to pay a decent fee for Colwill until such a time as he's nailed down a permanent starting position in the side and has shown he can deliver the goods on a regular basis. At the moment he's far more about raw natural ability than end product, and managers seem to find it difficult to accommodate such players. The six managers he's had at Cardiff certainly have and his international managers haven't seemed overly keen either. Nobody has seemed sure what to do with him.