Changing font colour

Hi,
I've looked at some posts on your site and followed the advice, however I can't get this to work.
I'd like to change the font colour of a range of cells depending on whether it contains a formula or text.
Initially all cells in the range contain a formula, but if a formula is overwritten I'd like the colour to change to remind the user that the cell no longer contains a formula.

Sub Lock_AllSnCells()
Dim aCell As Range
Dim bCell As Range
Dim cCell As Range
Set aRange = Range("AA7:AD36")
Set bRange = Range("BC7:BF36")
Set cRange = Range("BN7:BU36")
ActiveSheet.Unprotect
'--
'For Each aCell In aRange
' If Left(aCell.Formula, 1) <> "=" Then
' aCell.Font.ColorIndex = xlNone
' aCell.Font.Color = vbRed
' End If
'Next aCell
For Each cell In aRange
cell.Font.Color = vbBlue
Next cell
For i = 7 To 36
checkcell = Range("AA" & i).Formula Like "*[A-N]*"
If checkcell = True Then
Range("AA" & i).Font.Color = vbRed
End If
Next
aRange.Select
With Selection
Selection.Locked = True
Selection.FormulaHidden = True
End With
Set aCell = Nothing
Set aRange = Nothing

Ranges b and c are repeats of the above.

I've tried a couple of ways with the same result - the sub runs and changes the first cell then stops. No error message, just stops.

Any suggestions would be much appreciated.
Thanks