They are yellowing because of the heat............seriously, insulate between the rafters, and cover them with poly~I know fall and winter is coming, and then you'll be fine, but come spring....invest in some insulation, buy a stapler and slap the insulation between those rafters.....then cover the insulation with a layer of poly, Even go so far as to build a "dumy wall" where you stop insulating...you'll see a difference! I cant grow in my outdoor closet in the summer cause it's WAY too hot. It got over 90 in there, the res temps skyrocjeted and I got root rot, lost a whole crop do to poor ventilation/heat (my space is too small for a portable ac unit...But your set up is choice for fall winter crops!