The WMF Converter Pro allows you to replace colors in the vector based WMF files. Let's select a file, and click on the Replace Color button on the right of the Tools to display the Replace Color panel.

Set the color to be replaced by clicking on the left Pick up Color button, moving the cursor to the image area and selecting a color. Set the destination color by clicking on the right Pick up Color button.

Click between the Pick up Color buttons to make link between colors. This immediately modifies the image in corespondence with the new color settings.