[Fizzlemizz]

<Scripts>
	<OnLoad inherit="prepend">
		if BackdropTemplateMixin then -- if this is NOT a Classic type frame, add the backdrop mixin
			Mixin(self, BackdropTemplateMixin)
		end
		self:SetBackdropColor(1.0,1.0,1.0,1);
	</OnLoad>
	<OnMouseDown>
		if ( button == "LeftButton" ) then
			self:StartMoving();
		end
	</OnMouseDown>
	<OnMouseUp>
		self:StopMovingOrSizing();
	</OnMouseUp>
</Scripts>

I missed your OnLoad code.

if BackdropTemplateMixin then -- if this is NOT a Classic type frame, add the backdrop mixin
    Mixin(self, BackdropTemplateMixin)
end
self:SetBackdropColor(1.0,1.0,1.0,1);

Adding the mixin just configures the frame to use a backdrop. Once that is done, you've then have to actually add the backdrop to colour.

if BackdropTemplateMixin then -- if this is NOT a Classic type frame, add the backdrop mixin
    Mixin(self, BackdropTemplateMixin)
end
self:SetBackdrop( {... you have to find the actual backdrop information to put here } );
self:SetBackdropColor(1.0,1.0,1.0,1); -- and set the colour