OK:
HURRAY, now I understood you (after I run _genxtab on my table, because I never used it before)

FOR i = 1 TO thisform.GrdInvRes.ColumnCount
    ******
    **You must skip the columns that are not interesting
    ****
    dynamic_color = "IIF(xtabresult."+FIELD(i)+"='NEG',RGB(255,255,255),IIF(xtabresult."+FIELD(i)+" $ 'POS AGG',RGB(255,255,128),RGB(0,0,0)))"
    thisform.GrdInvRes.Columns(i).dynamicforecolor = dynamic_color
thisform.GrdInvRes.Refresh()