macro not working on sheet change

Afternoon All

I have an excel 2003 workbook with a macro in, the macro changes the colour of a cell based on its value. the macro works up to a point.

on the 1st sheet we have cells all linked to the contents of cells in other sheets
"screen1.jpg"

if I go to another sheet, and change the value from g to X the cell turns black, which is what the macro is meant to do.
"screen2.jpg"

if we then go back to the 1st sheet excel notices that the value has changed from G to X but has made the background white?
if I manually type in X it goes black,

Does anyone know anything I could be doing wrong here?

the macro is:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim oCell As Range
For Each oCell In Target
        Select Case oCell.Value
             Case Is = "R", "r"
                 oCell.Interior.Color = RGB(255, 0, 0)
             Case Is = "G", "g"
                 oCell.Interior.Color = RGB(0, 255, 0)
             Case Is = "A", "a"
                 oCell.Interior.Color = RGB(255, 102, 0)
             Case Is = "B", "b"
                 oCell.Interior.Color = RGB(0, 0, 255)
             Case Is = "x", "X"
                 oCell.Interior.Color = RGB(0, 0, 0)
             Case Is = "BRX", "brx"
                 oCell.Interior.Color = RGB(100, 100, 100)
             Case Else
                 oCell.Interior.ColorIndex = xlNone
         End Select
    Next oCell
End Sub

thanks in advance for any replies