OK, there are some distinctly odd things about the election, to the extent that I would agree that it would be good to see it run again more openly.
However, even given this, there doesn't seem to be a lot of evidence to suggest that Mousavi should have won (although I haven't looked into it in that much detail). The BBC even suggest that independent international pollsters came up with a similar 2-1 result toward Ahmadinejad compared to Mousavi a few weeks earlier with the others only getting a small proportion of the vote.
OK, there are vocal supporters of Mousavi, as there are for Ahmadinejad, but you cannot assume they are representative of the population.
I'm not Iranian and as such have no right to have any say in there elections, so I am not going to show my support for a politician who I have not seen any evidence to suggest that they would have won in a fair and democratic election, by adopting his colours. However, I do certainly agree that the elections should be re-run in a fair and open way.