probably need some npk additive, what nutes are you using? almost all nutes have a npk additive for those heavy bloomers, bubba kush is one of those heavy indica strains so I'm sure you plants are heavy and buds are sucking the nutes right out the leaves. yellowing is normal right before harvest but I think your still about 3 weeks from harvest, so maybe a little npk will help, but maybe they are ripening. how is your tricom development?