Function myRGB(r, g, b)

    Dim clr As Long, src As Range, sht As String, f, v

    If IsEmpty(r) Or IsEmpty(g) Or IsEmpty(b) Then
        clr = vbWhite
    Else
        clr = RGB(r, g, b)
    End If

    Set src = Application.ThisCell
    sht = src.Parent.Name

    f = "Changeit(""" & sht & """,""" & _
                  src.Address(False, False) & """," & clr & ")"
    src.Parent.Evaluate f
    myRGB = ""
End Function

Sub ChangeIt(sht, c, clr As Long)
    ThisWorkbook.Sheets(sht).Range(c).Interior.Color = clr
End Sub