Firstly, click on any bar of your chart. Then a sidebar named Format Data Series will appear. From the Series Options select Fill. You may see a lot of options under Fill. Select Solid fill. Then select a color you wanted to apply in the chart bars from the Theme Colors options.

On the Format tab under Chart Tools, do one of the following: Click Shape Fill to apply a different fill color, or a gradient, picture, or texture to the chart element. Click Shape Outline to change the color, weight, or style of the chart element.