In several gnome applications, when you click on a color widget to set a new color, if the RGB values of the new color are not extreme values like 0 or 1, the color that shows on the color widget icon (after you exit the color selection window) is "lighter" than the one chown in the color selection window. Example: go to the panel properties; click on the "Background" tab; click "Color" to enable color selection; click in the "Background color" widget on the bottom; select a new medium color like 0.5,0.5,0.5 and click "Ok". The color shown in the "Background color" widget is lighter than the one you selected; click on the widget again to compare.
According to "xmag" the two colors are the same (to try this, run "xmag", select the preview in the color dialog and the preview in the color button, then click the color in xmag that you want to see the numbers for).