[SDPhantom]

It doesn't look like FocusFrame even registers UNIT_TARGET for TargetofFocusFrame. It looks like you can hook TargetofFocus_Update() to accomplish this.

hooksecurefunc("TargetofFocus_Update",function(self)
    if not self then self=TargetofFocusFrame; end
 
    if UnitExists("focus-target") then
        if UnitIsPlayer("focus-target") then
--          Unit can exist, but if we haven't received data yet, UnitClass() can return nil
            local color=RAID_CLASS_COLORS[select(2,UnitClass("focus-target")) or ""];
 
            if color then
                self.healthbar:SetStatusBarColor(color.r,color.g,color.b);
                self.healthbar.lockColor=true;
                return;--   Force out of function
            end
        end
    end
 
--  If we reach here by any way, we wern't able to change the color
    self.healthbar.lockColor=false;
end)
 
--  Update handler pointer to the hooked function
TargetofTargetFrame:SetScript("OnUpdate",TargetofFocus_Update);

Note not only is TargetofFocus_Update() the OnUpdate handler for TargetofFocusFrame, but is also called with no arguments from FocusFrame's OnUpdate handler.