>ThisForm.grdInv.SetAll("DynamicBackColor","IIF(threshold<1000,RGB(255,255,0),RGB(255,255,255)","Column")

A. You're missing a ")" (in bold below)
B. You may need to refresh the grid a couple times (like in ARCC())
C. You may want to qualify threshhold with an Alias.

ThisForm.grdInv.SetAll("DynamicBackColor","IIF(" + alias() + ".threshold<1000, RGB(255,255,0), RGB(255,255,255))","Column")