Hi @JustJoe,
You can't set the background of only one bar. You. can however specify a custom set of colors for the fill (background) of every bar:
<RadzenBarSeries Fills="@(new [] {"red", "green"})" ... />
RadzenChart will cycle the colors specified in the Fills property when rendering the bars. I think you can use this as a workaround and specify unique "fill" for the desired series.