I think white or light grey curtains are better, as lighter colors absorb less heat.
Use of under-lay or jambolon (available at carpet shop) under floor carpet is also very effective.
Foaming around evaporator coil (but not the bottom side) is a must. (Many AC technicians don't do that).
Air exhaust vents flaps behind rear bumper are missing in most old cars, due to which hot air enters the cabin.