I just looked into how this is done, the Poll was a community contribution by Christian Fredh, so having not implemented it myself I was not sure how the bars worked.
It appears that the code first looks for an image named pollresult.jpg in your skin folder (presumably this could be a 1 pixel image of any color. It then sets a width and height to size the bar using this image.
If the file pollresult.jpg is not found in the skin folder, then it checks Web.config, you could add a setting there with the key "PollResultColorIfNoImage" and the value must be a named color from the web pallette blue, green black etc. Unfortunately it expects only named colors and isn't coded to handle hex notation.
If niether the image nor the web.config setting is found it defaults to blue.
skin folder = /Data/Sites/[SiteID]/skins/[skinname]
Hope it helps,