To actually select a color, you should use the 4th color palette. The first three palettes modify this forth one.
By choosing a color in the first palette, you populate this color in the forth one using black-color-white gradient.
By choosing a color in the second or third palettes, the 4th is changed to a gradient from this color to the selected color.
Moreover, clicking on the selected color has the same effect as choosing a color using the first palette.
Finally, you can modify the Red, Green and Blue values of the selected color with the mini scroll bars.
Report bugs to: fvwm-themes-devel@lists.sourceforge.net.