Isn't sunlight much more powerful in terms of energy than these lights? Couldn't we use a "one-way" mirror to reflect the led/lights inside, and still allow sunlight in during the day? The additional reflections inside the greenhouse might more than make up for the "shade" provided by the "one-way" mirror?

(now, AFAIK there are no actual one-way mirrors, simply semi transparent (both directions) - question is if we cut 70-80% of the leds escaping, would 20-30% sunlight entering be enough?)

LEDs are supplemental. Sunlight gives you about 1kW/m² for free, it's hard to beat that. So anything that sacrifices a large fraction of sunlight is probably a non-starter, especially during the colder seasons where you also need it to keep the glasshouse warm.

I suppose you could always go for semi-transparent - so not reflect but absorb a fraction of the light?

The problem would be that those narrow band glass filters would also block those narrow bands in daylight needed by the plants. They would have to have those LEDs on 24/7 which means a greenhouse is no better than a basement with no daylight.

I think they should simply use curtains or shutters that close at night. I'm surprised a modern country like Netherlands has no laws regarding light pollution.

I was actually talking about glasses for humans, but I will roll with your argument anyway:

Plants are not narrow-band absorbers. They can utilize rather wide ranges of red and blue light. So if you blocked out a very narrow band there they would still get most of the sunlight. During the night they would simply harvest photons from those narrow bands, they don't care. But the photons wouldn't escape the glasshouse.

But yes, shutters are probably much cheaper than narrowband light sources and matching filters.

Or just curtains?