(Using scenebuilder-1_0-ea-b12-windows-i586.zip)

How to re-produce:

1. Launch SceneBuilder and select "Empty"
2. Drag a Rectangle and drop it into the default Anchor Pane
3. Click the "Color Tab" in the Inspector panel on the right
4. Choose LinearGradient from the "paint type" combobox for "fill (Paint)"
5. Click on the small white block next to "start color"
5. In the color template(color chooser), drag the indicator of the 3 R/G/B
   sliders to left-most, setting the 3 values to 0(which means all are
   set to black)
6. Click Layout->Show Preview, you will see in the Preview window the
   new colors are not reflected(old colors are still there).