heat for one and its gonna create hot spots= frying ur plants to death
now let roll on to the yellowin subject....................... u still got some green goin on there....usually there be some heads in here whose plants are all yellow in shit ..... they way it looks it can be two things:
if anything very little ......it will just be better if u can paint the wall flat white which a gal of paint set u bac bout 5 bucks or so and its just as efficient as mylar well not the best as mylar but about 5-10% less reflective ............