Each of the dialogs displaying the 256-color VTS palette initially operates in "free color mode". As you run the mouse pointer over the swatches in the palette and over the graphics on the currently displayed page, the sample area below the palette picks up and displays the color under the mouse pointer. This behavior enables you to rapidly match your page's color scheme.
To select a color (either from the palette or from your graphics page), click on the desired color. The selected color will be set in the sample area, and free color mode will be turned off (i.e. moving the mouse pointer across the graphics page or the color palette will no longer pick up and display the underlying colors in the sample area). This does not mean that you cannot select an alternate color but, you are limited to the 256 colors appearing in the color palette. Any color you click in the color palette will be set in the sample area.