Change to :

DynamicBackColor is a Character expression

thisForm.pgfpagerefresh1.page4.Grdtmdataentry1.SetAll("DynamicBackColor", "thisform.zGridColor()", "Column" )	

The evaluation of the DynamicBackColor expression must return a Color value

* cColor = "RGB(255,255,255)"	 && useless
DO CASE
   CASE taskcomp	                  && done... GREEN
      RETURN RGB(215,255,215)
   CASE EMPTY(taskend) AND taskbeg <= DATE() + 9
      RETURN IIF(taskbeg <= DATE() + 2,RGB(255,222,213),RGB(250,253,193))
   OTHERWISE
      RETURN  RGB(255,255,255)  	&& white
ENDCASE

If you use a IIF(,IIF(,... expression you can remove the zGridColor method.