Just add this CSS to the end of your templates default.css
/******************** Poll Module Result Bar Colors Begin *******************/
div.pollResultStatsRow:nth-child(5n+0) {
background-color: #3498db !important;
}
div.pollResultStatsRow:nth-child(5n+1) {
background-color: #e74c3c !important;
}
div.pollResultStatsRow:nth-child(5n+2) {
background-color: #1bc5a4 !important;
}
div.pollResultStatsRow:nth-child(5n+3) {
background-color: #9b59b6 !important;
}
div.pollResultStatsRow:nth-child(5n+4) {
background-color: #e67e22 !important;
}
/******************** Poll Module Result Bar Colors End *******************/
Feel free to change the colors, or to add more. The css to repeat should be obvious. Change the above css to reflect how many different colors you want to cycle through. xn is the counter where x = the number of colors you will use, and it is common to all the different child selectors. Then increment the counter as shown above until you have the css for all the colors.