Do you think Colin Powell was a major factor?
In my view, no, it was definitely not a major factor, if a factor at all. McCain had the endorsement of, I believe, five former Secretaries of State. That had no impact either.
Endorsements usually mean very little. Voters do not make up their minds on who to vote for based on endorsements. Endorsements are only useful for the publicity they may generate for a fleeting moment, or, in the case of Powell, becoming a topic of discussion on the news shows for a few days.
The Powell endorsement did not bring wavering independents to Obama. The desire for change and Obama's perceived ability to better deal with the faltering economy did that. I doubt as well it had any impact with Republicans.