Balls okay, so I figured I needed to use VB to get this to work, and it kind of does (I'm not a programmer) so I used:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim WatchRange As Range
Dim CellVal As String
If Target.Cells.Count > 1 Then Exit Sub
If Target = "" Then Exit Sub
CellVal = Target
Set WatchRange = Range("b3:m3") 'change to suit

If Not Intersect(Target, WatchRange) Is Nothing Then
Select Case CellVal
Case "Y"
Target.EntireRow.Interior.ColorIndex = 5
End Select
End If
End Sub

HOWEVER, this doesn't colour the range B3:M3 but rather the entire row, also when cell M3="Y" is colours the cell, but when M3="" it leaves it coloured... Any idea's where I've cocked up?