I did not implement the Poll feature myself, but looking at the code here is what it does.
1. It looks for a file named pollresult.gif in the skin folder (I think it expects like a 1 pixel gif of whatever color you want.)
2. If it does not find the gif file, it checks Web.config for a setting named PollResultColorIfNoImage
3. If no value is provided in the setting it uses blue
I'm thinking a better solution will be to add a color setting so the bar color can be specified using our color picker.