Millennial Rites
The Quantum Archangel
Slipback
I always felt defensive about Colin's time on the show. He was constantly attacked by fans (and BBC controllers) who really didn't give him credit for trying something different. It would have been quite interesting to see where he took the character in the long term. One thing I have to say about Colin's doctor is that he's easily the best out of all the audio doctors (especially when paired with Evelyn Smyth).
Revelation of the Daleks
The Two Doctors
Attack of the Cybermen
The Trial of a Time Lord parts 13 - 14 (The Ultimate Foe)
The Trial of a Time Lord parts 5 - 8 (Mindwarp)
The Twin Dilemma
Attack of the Cybermen
Vengeance on Varos
The Mark of the Rani
The Two Doctors
Timelash
Revelation of the Daleks
The Trial of a Time Lord parts 1 - 4 (The Mysterious Planet)
The Trial of a Time Lord parts 5 - 8 (Mindwarp)
The Trial of a Time Lord parts 9 - 12 (Terror of the Vervoids)
The Trial of a Time Lord parts 13 - 14 (The Ultimate Foe)