I did get a can of leftover varnish and though it will bring out the colour of the bamboo.
Turned out to be very shiny finish that I dont like. Looks too artificial.
I found that the shine in the inner part is reflecting light pretty well - not loosing too much light in there.
But I'm considering to sand the outer shell to get back their natural look.
I'll update this I'ble with more pictures for comparison if I decide to do that.
--------------------- The Modified one:
This one takes a bit more to it. I used a thin fabric that has the perfect translucent property: let trough enough light to work and not too bright even to see directly at. Cut to size and glue them to the inner side of the shade. Also a bit tricky with my big hands.