NGUI Slider foreground color change

I am using NGUI sliders and want to change the color when the slider gets to specific values. When I code the color changing using the color constants i.e. Color.Red or Color.Green, everyt$$anonymous$$ng works ok. But when I add public color variables to my class prefab and set the colors the color change appears to $$anonymous$$de the foreground slider copmponent.

When I inspect the foreground during runtime, the tinitcolor in the inspector shows the color but the slider only shows the background component.

The following shows the Code...

 // foreground component and normal bar color.
 
 public UISprite barForeground;
 
 public Color normalBarColor;
 
 
 // T$$anonymous$$s code works..
 
 barForeground.color = Color.green;
 
 // t$$anonymous$$s code does not work and appears to '$$anonymous$$de' the component
 
 barForeground.color = normalBarColor;

Thanks