Private Sub CommandButton1_Click()

Dim fc As Integer, fr As Integer
fc = 1
fr = 1
Dim lr As Integer
lr = Sheets("sheet2").Range(Cells(fr, fc), Cells(fr, fc).End(xlDown)).Rows.Count

Dim sc As Integer, sr As Integer
sc = 1: sr = 1

For i = fr To lr

If Sheets("sheet2").Cells(fr, fc) <> Sheets("sheet3").Cells(sr, sc) Then
Sheets("sheet3").Cells(sr, sc).Interior.ColorIndex = 3
End If
fr = fr + 1
sr = sr + 1
Next

End Sub