Hi ... try this code which also resets to normal cell when you delete the wrongly entered data in cells......... and only when you enter data it colors it yellow..... 
Private Sub Worksheet_Change(ByVal Target As Range)
Set MyRange = Range("A1:A1000")
    For Each Cell In MyRange
        If Cell.Value < 1 Then
        Cell.Interior.ColorIndex = x1None
        End If

        If Cell.Value >= 1 Then
        Cell.Interior.ColorIndex = 6
        End If
    Next

Set MyRange2 = Range("B1:B1000")
    For Each Cell In MyRange2
        If Cell.Value < 1 Then
        Cell.Interior.ColorIndex = x1None
        End If

        If Cell.Value >= 1 Then
        Cell.Interior.ColorIndex = 6
        End If
    Next

Set MyRange3 = Range("C1:C1000")

     For Each Cell In MyRange3
        If Cell.Value < 1 Then
        Cell.Interior.ColorIndex = x1None
        End If

        If Cell.Value >= 1 Then
        Cell.Interior.ColorIndex = 6
        End If
     Next

End Sub