There's a couple ways I've done it.
1. Paint the vision block gloss black.
2. Paint the block flat black and then cover with Micro Krystal Kleer. This adds a little depth.
3. Use black acetate film cut to size and attached with white glue. I did this on an AFV Club Scimitar with tremendous results.
For sights or periscopes (like the Marder III), I paint the periscope lens opening with Testors chrome silver, and when dry, cover it with a small drop of Tamiya or Gunze clear green acrylic paint.
Gip Winecoff