protected void Calendar1_DayRender(object sender, DayRenderEventArgs e)
    {
        if (e.Day.IsSelected == true)
        {
            int color = SelectedDates[e.Day.Date];

            byte[] values = BitConverter.GetBytes(color);
            if (!BitConverter.IsLittleEndian) Array.Reverse(values);

            Color myColor = Color.FromArgb(values[0], values[1], values[2]);
            e.Cell.BackColor = myColor;
        }
    }

reference :
http://stackoverflow.com/questions/6131438/mapping-an-integer-to-an-rgb-color-in-c-sharp