Hi!
That is not set by the plugin, it seems like you use some sort of a custom select script that sets it. Try using this custom CSS to reset it to any color:
div.asp_sb fieldset div.select2-container.select2-dropdown-open,
div.asp_sb fieldset div.select2-container.select2-dropdown-open a,
div.asp_sb fieldset div.select2-container:hover,
div.asp_sb fieldset div.select2-container:hover a {
background: red !important;
}