Colin is an excellent actor. The producers were, intentionally or not, basically killing the show at that point. Sylvester did not get his entire planned arc (which is why 'hand of omega' seems so strange, it was headed in the direction of the shows that were cut). The US movie .... well, the less said about it the better.
JNT was trying to kill off the series, that is why Colin's Doctor is such a mess. Colin did a great job with what he was saddled with.
Sylvester's run started off trying to dig out of the hole JNT had dug for Colin. As the series progressed, it got really good. I loved the dark mysterious Doctor who was there with Rasilon and Omega at the start of Timelord society. We got hints that made us want to know more, was the Doctor even older and more powerful then we had seen? The voiceover that was added to Survival (the last broadcast story)
"There are worlds out there where the sky is burning, the sea's asleep, and the rivers dream. People made of smoke, and cities made of song. Somewhere there's danger, somewhere there’s injustice, and somewhere else the tea's getting cold. Come on, Ace — we’ve got work to do!" was added after the story was shot but before the final episode aired. It was done to give a "more suitable conclusion".
I agree, the American harlequin romance doctor movie was crap.