What was the error? If the healthbar is not a StatusBar widget or the panels Onload fires before STUF is loaded that will cause errors as mentioned in Phanx's post.
It looks like the healthbar is not a statusbar but several seperate frames so my best guess would be:
Code:
hooksecurefunc(Stuf.units.target.hpbar.bar, "SetVertexColor", function(self, ...)
self.bg:SetVertexColor(...)
end)