Any help is appreciated.

I am running into trouble trying to write a Macro that does the
following:

* Affects all workbook tabs (i.e. worksheets).
* Zeros out all numeric constants.
* Color codes the following
* Named cells (gray background)

What I've got so far (see below) gives me the error "1004
Application-defined or object-defined error". I think that it has to do
with not being able to get a proper range reference to loop over cells
on a given worksheet but I am at a loss.

Sub ColorCodeAndZeroOut()
For i = 1 To Worksheets.Count
On Error GoTo ErrorHandler

For Each n In LastCell(Worksheets(i)).Cells

If IsEmpty(n.Name.Name) Then
n.Interior.ColorIndex = 0 'White
Else
n.Interior.ColorIndex = 15 'Gray
End If

If IsNumeric(n) And Not n.HasFormula Then
If n.Value < 0 Then
n.Value = 0
ProtectedCell:
End If
End If
Next n

ErrorHandler:
If Err = 1005 Then
Resume ProtectedCell
Else
MsgBox Err.Number & " " & Err.Description & " " &
Err.Source

End If

Next i
End Sub

Function LastCell(ws As Worksheet) As Range
Dim LastRow&, LastCol%

' Error-handling is here in case there is not any
' data in the worksheet

On Error Resume Next

With ws

' Find the last real row

LastRow& = .Cells.Find(What:="*", _
SearchDirection:=xlPrevious, _
SearchOrder:=xlByRows).Row

' Find the last real column

LastCol% = .Cells.Find(What:="*", _
SearchDirection:=xlPrevious, _
SearchOrder:=xlByColumns).Column

End With

' Finally, initialize a Range object variable for
' the last populated row.

Set LastCell = ws.Cells(LastRow&, LastCol%)

End Function