In current version of Google Charts, you cannot pass palette colors with an opacity other than 1. This free template shows you a hack: when the chart is rendered, we map the opaque colors passed in the palette to their equivalent semi-transparent colors, as they appear in current palette (defined by Microsoft). For instance, our first opaque color is red, and #ff0000 is mapped to rgba(255, 0, 0, 0.59). Both #ff0000 and rgb(255, 0, 0) are codes for red. However, the rgba function adds a last alpha or opacity value of 0.59, for a 59% transparency. You can see the gridlines through the bars, which shows the colors are indeed semi-transparent.