highlight cells in column 'G' that contains the word null

Try:

Sub FillCell()
    Application.ScreenUpdating = False
    Dim LastRow As Long
    LastRow = Cells.Find("*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
    Dim rng As Range
    For Each rng In Range("G2:G" & LastRow)
        If rng = "NULL" Then
            rng.Interior.ColorIndex = 3
        End If
    Next rng
    Application.ScreenUpdating = True
End Sub

Tony, you are correct, however, it appears the OP wants a VB solution

Hi,
here's another alternative elaborating with the evaluate function:

Sub BerlanActivesheetProcedure()
    Range(Join(Filter(Evaluate(Replace("transpose(if(@=""NULL"",address(row(@),column(@)),""#""))", "@", Range("G1", Range("G" & Rows.Count).End(xlUp)).Address)), "#", 0), ",")).Interior.ColorIndex = 3
End Sub

Otherwise, in addition to Mumps1's suggested approach, you could also use autofilter or the find+findnext function.

Kind regards,
berlan