Hi,
The ux:Value property is not applying when the property is not set in the instantiation of the class.
The BackGround color should be #FFFFFF when no value is set like this:
<float4 ux:Property="BackgroundColor" ux:Value="#FFFFFF" />
And instantiated as such:
<MyButton />
However the MyButton component has no background until I set it explicitly again in the instantiation like this:
<MyButton BackgroundColor="#FFF"/>