Please keep in mind: If individual points in a chart with multiple series are colorized manually, these manual changes will be discarded, if large-scale changes to the data are made afterwards. For this reason the individual color customization should be made after editing the diagram.
For colorization, please use native tools in PowerPoint®. To colorize an element individually, select it with a right click and click on the button „Fill“. A dropdown menu will open, that shows the color scheme of the master and its shading colors.
Choose one of these colors or click on „More Fill Colors“. In the „custom“ tab you have the option to define RGB values to set an exact color value.