[Fizzlemizz]

Something like:

local Color = { r=1, g=0, b=0 } -- red for any hostile
if UnitExists("target") then
	if not UnitCanAttack("player", "target") then
		local _, Class = UnitClass("target")
		Color = RAID_CLASS_COLORS[Class] or {r = 1, g = 1, b = 1}
	end
	self.bg:SetVertexColor(Color.r, Color.g, Color.b, self.bg:GetAlpha())
else
	self.bg:Hide() -- or set it to some generic colour
end