Gridview Cell Color needs to change based on CheckBox checked in asp.net/c# keep getting errors

I have a Gridview (ASP.NET / C#) that currently changes color based on dates provided by the user. Past Due is Red, Future or Current Date is white (Which Works Nicely!). I have also provided a checkbox in the gridview cell that designates "Complete". If that checkbox is checked, i want it to override and dates and change the cell color to Black. So far the errors i keep getting are either, object not set to an instance of an object when i have tried it under "RowDataBound" or now with the current code, i have tried putting it under "OnCheckedChanged" which now has prevented any changes to the checkbox.
The data is stored in the database as Bit. Update Command sets parameters as Boolean.

Any ideas on how to get this to work?

ASP.NET markup:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataSourceID="SqlDataSource1"
     GridView1_RowUpdating="true" DataKeyNames="ID" OnRowUpdating="Page_Load"
     BackColor="#CCCCCC" BorderColor="#999999" BorderStyle="Solid"
     BorderWidth="3px" CellPadding="4" CellSpacing="2" ForeColor="Black"
     OnRowDataBound="GridView1_RowDataBound">
    <Columns>
        <asp:TemplateField HeaderText="Verify Info Prod & Maturity Level" SortExpression="IPMaturity">
            <EditItemTemplate>
                <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("IPMaturity", "{0:MM/dd/yyyy}") %>' Width="75"></asp:TextBox>
                <asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="true" Text="Complete" Enabled="false" OnCheckedChanged="CheckBox1_CheckedChanged" 
                     Checked='<%# Bind ("CheckBox1") %>'></asp:CheckBox>
            </EditItemTemplate>
            <ItemTemplate>
                <asp:Label ID="lblIPMat" runat="server" Text='<%# Eval("IPMaturity", "{0:MM/dd/yyyy}") %>' Width="75"></asp:Label>
            </ItemTemplate>
            <HeaderStyle CssClass="verticaltext" Height="140px" Width="88px" HorizontalAlign="Center" VerticalAlign="Bottom" />
        </asp:TemplateField>

C# code behind:

protected void CheckBox1_CheckedChanged(object sender, EventArgs e) //from checkbox
{
     CheckBox chkItem = null;
     //Boolean chkb = Convert.ToBoolean(DataBinder.Eval(e.Row.DataItem, "CheckBox1"));

     foreach (GridViewRow grRow in GridView1.Rows)
     {
         if (grRow.RowType == DataControlRowType.DataRow)
         {
             chkItem = (CheckBox)grRow.Cells[4].FindControl("CheckBox1");
             bool bl = chkItem.Checked;

             if (bl == true)
             {
                 grRow.BackColor = Color.Black;
             }
         }
    }
}

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)  // Changes color of cell based on date
    {
        string ipMaturity = Convert.ToString(DataBinder.Eval(e.Row.DataItem, "IPMaturity"));
        DateTime? date1 = !string.IsNullOrEmpty(ipMaturity) ? Convert.ToDateTime(ipMaturity) : (DateTime?)null;

        if (date1.HasValue && date1 >= DateTime.Now)
            e.Row.Cells[4].BackColor = System.Drawing.Color.White;

        if (date1.HasValue && date1 < DateTime.Now)
            e.Row.Cells[4].BackColor = System.Drawing.Color.Red;
    }
}