yeah, for some reason the leaves always seem just a little too short . . .
actually, sometimes I like to think it's my wonderfully gardening skills that result in the head size out-growing the leaves , , , makes me feel better after all that work (g)
if the shade cloth also shades the leaves, that wouldn't be especially good - except in the case where it make help keep the plants cooler as the hot season sets in.
the greenish/yellowish is chlorophyll development - and a total light block is best at keeping them pure white. I've used (scrap cloth) squares - tied the corners with string - a real pita for more than just a few plants.
plan B is: iffin' I can't get them covered, eat 'em - in our area we have about a 2-3 week window where the heads are "of size" until the florets start to separate - so harvesting them early at a smaller size gives us a longer 'fresh season'