I found a site (
http://www.diatex.fr/uk/produits/agri/ombrage.htm) selling shade nets. And this is what they have to say or warn (darn, why now?) before buying their shade nets:
Caution
Green and black shade nets behave like filters. Essential radiation for photosynthesis is reduced.
Thus, the growth is reduced. A green and black shade net decreases the light’s quantity and spectrum quality.
White shade nets :
They decrease only light’s quantity, without altering luminous’ spectrum quality. As a consequence, the plant’s growth is faster with a white shade net.