Mike:
By the Adj. Genl's report, Colvin was one of the original members of A.J. May's Co. A of the Fifth, enlising on 10/25/61 in Prestonsburg, just a couple of days after Co. A had their fight with Col. Harris and his 2nd OVI at West Liberty. Colvin remained there until 10/20/62 when the 12-month men of the Fifth were disbanded at Hazel Green and vicinity. He was elected Capt. of Co. F upon reorganization of the Fifth on 9/30/62. There evidently weren't sufficient men to form the company and Gen. Marshall on 11/14/62 ordered Co. F consolidated with Capt. Cope's Co. I, which then became Co. D of the consolidated Fifth. After Cope was elected capt. of Co. I, Colvin evidently didn't hang around once he was out of a command because he doesn't appear on the Co. I muster rolls. There was also a Wm. Colvin who enlisted 9/22/62 when Co. F was formed (a brother or cousin?) but who, like Capt. Colvin, does not appear on Cope's roster following the consolidation of Cos. F and I. So Colvin was probably a part of the Fifth from October '61 through mid to late November '62. After that date he no longer appears on any Kentucky Confederate roster I can find.
Hope this helps and becomes part of a fruitful search for you.
Randall