Excel charts do not support color transparency. When you apply a color to a bar or column chart series, the color must be a solid color. Although the Fill Effects dialog box includes Transparency controls, these controls are disabled.
This tip describes how to make a bar or column series transparent by pasting a picture. It could be an interesting and visual approach when creating combination charts that plot 2 different types of chart on 2 y-axes. Of stead of the common line chart super-imposed on a column chart, you can now have a column chart super-imposed on an area chart