﴿ وَلَئِنْ أَرْسَلْنَا رِيحًۭا فَرَأَوْهُ مُصْفَرًّۭا لَّظَلُّوا۟ مِنۢ بَعْدِهِۦ يَكْفُرُونَ ﴾

“But thus it is: if We send a wind [that scorches their land], and they see it turn yellow, they begin, after that [erstwhile joy], to deny the truth [of Our almightiness and grace]!”

But if wa-la-in the lām is for oaths We unleash a different wind that damages vegetation and they see it turn yellow they would begin la-zallū is the response to the conditional oath after that that is after its turning yellow to disbelieve to deny the grace of having been given rain.